Hi -

 

I’m thinking about getting a fast prime to go with my zooms which are fixed at f4. I’m thinking of the Sony SEL 35 mm f1.8. I realize that on my (ordered) a6300 that the 35mm will be equivalent to about 52mm in full frame terms, but… does the f1.8 have an equivalency also? Does it convert to like a f2.8 or does it stay f1.8 because the ratio to the lens length is the same? (“IF” I understood that correctly?)

 

Also, my lower end zoom is the Zeiss f4 16-70. Does my thinking make sense in wanting this faster f1.8 lens?

 

Thanks All

The image will be identical to that from a FF camera, expect that it will be cropped. So, it will seem to be around 52mm focal length, and have same brightness (Transmission?) and most other image qualities. It will still behave like the 35mm lens, just cropped.

 

IIRC the "brightness" will be identical to an F1.8 APS-C lens, since the additional light which is lost, is lost due to the mechanical size of the sensor and not a dimension of the lens.

 

 

 

 

In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture^[1]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.^[2] It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. The number is commonly notated using a hooked f, i.e. f/N, where N is the f-number.

https://en.wikipedia.org/wiki/F-number

You can find some answer here (Mirrolessons web site):

 

http://www.mirrorlessons.com/2016/02/08/aperture-equivalent-sensor-size/

 

 

Envoyé de mon iPhone à l'aide de Tapatalk

Thanks Noaka and timde! I got quite an education today! I can't say I understood everything I read, but I enjoyed it. 

 

Many thanks!

when you multiply crop factor you also have to multiply the f/stop. if you get a 50mm f/1.8 on an a6000, it will give the same amount of exposure, but the depth of field will not. to get the correct f/stop value for the depth of field you simply multiply the f/stop by the crop factor.

sony's aps-c sensors are about 1.53x crop. so:

 

f/1.8 x 1.53 = 2.754

 

meaning the 50mm f/1.8 will give the exposure of a f/1.8 full frame lens, but the depth of field of an f/2.754 lens.

so a Sony rx10 Mark II, which has a f/2.8 lens, is actually

 

[f/2.8 x 2.73 = 7.644]

 

a f/7.6 in terms of depth of field, but f/2.8 in terms of exposure

when you multiply crop factor you also have to multiply the f/stop. if you get a 50mm f/1.8 on an a6000, it will give the same amount of exposure, but the depth of field will not. to get the correct f/stop value for the depth of field you simply multiply the f/stop by the crop factor.

sony's aps-c sensors are about 1.53x crop. so:

 

f/1.8 x 1.53 = 2.754

 

meaning the 50mm f/1.8 will give the exposure of a f/1.8 full frame lens, but the depth of field of an f/2.754 lens.

 

 

 

THAT makes sense to me! Especially after reading the article that Noaka referred me to which talks a lot about depth of field. Yes, I am interested in good DOF, but I'm looking for a lens to use at night and indoors when my f/4 lenses can't give me the exposure that I want without pumping up the ISO. I realize there will always be a trade-off, but at least I'll have the shot. Thank you for the clarity of your explanation!

when you multiply crop factor you also have to multiply the

f/stop. if you get a 50mm f/1.8 on an a6000, it will give the

same amount of exposure, but the depth of field will not. to

get the correct f/stop value for the depth of field you simply

multiply the f/stop by the crop factor.

sony's aps-c sensors are about 1.53x crop. so:

 

f/1.8 x 1.53 = 2.754

 

meaning the 50mm f/1.8 will give the exposure of a f/1.8 full

frame lens, but the depth of field of an f/2.754 lens.

I would divide by the crop factor, not multiply by it.

 

Optics is a world of math, and some concepts may seem

contrary to what one has studied. So you hafta get hold

of the concepts first, and only THEN apply the formulae.

Otherwise, one may misapply a formula that belongs to a

whole different circumstance.

 

If two lenses have the same field of view, but this is

on differing format sizes, then and only then will the

smaller format lens have a shorter FL and have greater

DOF at the same numeric f/stop as a longer lens on a

larger format. But this is not the circumstance in this

thread. The circumstance of THIS thread [per the OP] is

as follows immediately below:

 

If the FL of the lens is constant, then a given numeric

f-stop will deliver LESS DOF when you crop the picture.

Putting the very same lens onto a smaller format camera

is simply cropping. [same size finished print dimensions].

 

Take a portrait with lotsa deep DOF. Blow it up big and

almost stick your nose on it when judging limits of DOF,

IOW limits of acceptable looking sharpness. OK, now

blow it up even bigger, and similarly stick your nose on it.

Under this closer inspection, you will find less DOF. It

will have narrower limits of acceptable looking sharpness.

 

 

The bigger blow up appears to have less DOF if inspected

equally critically as the smaller blow up. This should not

seem remarkable. It should seem intuitive, even obvious.

Thankfully, the audience tends to stand somewhat further

away from larger prints than from smaller ones.

 

However, now we gonna cut the edges offa the bigger print,

right down to the same dimensions as that smaller blow up.

And hey what a coincidence. The trimmed down print is a

head-and shoulders view, while the untrimmed [and less

blown up] print is a half length view [same shot, but simply

a tighter crop].

 

Same lens, distance, f-stop, etc but the half length view

[basically full frame] shows more DOF than the head-and-

shoulders view [which is cropped by trimming down the print

dimensions to match the print of the half length portrait].

It's the identical thing as using the same 50/1.8 on both a

FF and an APSC camera, to shoot the same portrait from the

same distance. The FF shoots a half length, and the APSC

shoots a head-and-shoulders.

 

Remember, Circles of Confusion in the acceptable focus

range are getting bigger as they image subject detail thaz

further away from true exact focus. Increasing final view

magnification further increases the size of all those COC.

 

For practical purposes, the truly exactly focused details do

not form COC, but remain as true points, not circles, and

so the exactly focused details do not grow softer looking

as final view magnification increases.

 

-----------------------------------------------------

 

P.S. I hereby apologize to all those who were emotionally

traumatized by the suggestion of shooting a portrait with

lotsa deep DOF.

when you multiply crop factor you also have to multiply the f/stop. if you get a 50mm f/1.8 on an a6000, it will give the same amount of exposure, but the depth of field will not. to get the correct f/stop value for the depth of field you simply multiply the f/stop by the crop factor.

sony's aps-c sensors are about 1.53x crop. so:

 

f/1.8 x 1.53 = 2.754

 

meaning the 50mm f/1.8 will give the exposure of a f/1.8 full frame lens, but the depth of field of an f/2.754 lens.

 

 

There is a lot confusion and misinformation on the internet around this topic, it's actually hard to be sure of anything.  So I may be wrong, and please correct me if that's the case.

 

I've seen that statement before, that it "gives the same exposure, but not the same DOF".  That never made sense to me, how can it change one but not the other?  So I dug deeper.

 

My understanding, is that chris's info above is correct.  BUT!  It is only correct in one specific situation.  So allow me to try to add some detail.

 

This formula: f/1.8 x 1.53 = 2.754

Is making the assumption that the composition of the subject of the photo is constant.  Meaning that we are basically "factoring out" the crop factor.

In order to do that, physically speaking, with all other things being equal; we have to move the camera further away from the subject.

It is that change in focal distance that changes the DOF.  It has nothing to do with aperture or crop factor or anything else.

 

If instead of keeping composition constant, you were to keep the focal distance constant, then there would be zero change in DOF.  And so the formula above would be wrong.

If you use that 50/1.8 lens on a FF camera and take a shot, you will have "X" DOF.  If you then take the same lens and put it on a crop body, and don't change ANYTHING else, don't change focus, don't move the camera, don't change anything, and take a shot, you will have the EXACT same DOF.

Except of course that the image will now be literally cropped, your subject's head will probably be cut off.  To fix that you do have to change something, either use a wider lens or move the camera further back, and either those changes will also alter the DOF.

 

There are a number of blog's and youtube videos that demonstrate this, I think Tony Northrup demo'd it once.

 

 

So, going back to the original question: "Does the APS-C aperture stay the same in Full Frame terms?"

The answer is Yes.  Aperture and sensor size have nothing to do with each other.  So everything is the same no matter how you change one or the other.

 

However, it is assumed that when you ask about Aperture, you are really asking about DOF.    

Even then, the answer is still Yes.  Sensor size and DOF have nothing to do with each other, so changing one does not change the other.

 

It turns out though that there is an indirect relationship.  The process of factoring out the crop factor, forces the photographer to make changes that do have the unavoidable side effect of altering DOF.

But if you wanted to get a really tight shot anyway, then there's no harm in using the crop body vs ff.  You'll still get the same exposure, same dof, same bokeh.  The only thing missing will be all the stuff around the subject.

Its more to do with the viewing size and viewing distance, a FF image is enlarged less than an APS-C image, and that has an impact on how much DOF you get. As Golem explains in many more words.

 

The actual image on the sensor plane is unchanged.

Tony Northrup has several you tube videos that go into great detail on this subject. Worth looking into.

I don't see what you mean by being confused. it makes perfect sense to me. yes you are correct, he equation I provided (if you can call it that) assumes that you will be using the aps-c lens in the same way as its full frame counter part.

 

meaning if you use 50mm at f/1.8 lens on a full frame camera like a Sony a7 for a closeup, and a cropped 50mm at f/1.8 on Soy a6000 also for a closeup, meaning both cameras framed on the same subject, in the same way, then the math holds. that's the laws of physics.

 

that's the way you would expect an average person to use these lenses though.

 

in short, there are many factors that go into a much LARGER equation if you really want to know everything about optics. there is the size of the image plane, the f/stop of the lens, the distance to the subject, among others. I provided my simplified version of the equation because I wanted to make it easier for the average person. for deep math thinkers there are more detailed explanations at Film Maker IQ. here are some videos.

 

https://youtu.be/MytCfECfqWc

https://youtu.be/1YIvvXxsR5Y

https://youtu.be/CGGUXAMliqM

https://youtu.be/lte9pa3RtUk

https://youtu.be/f5zN6NVx-hY

https://youtu.be/YDbUIfB5YUc

https://youtu.be/DtDotqLx6nA

https://youtu.be/6Im4W_9blhY

I don't see what you mean by being confused. it makes

perfect sense to me. yes you are correct, he equation

I provided (if you can call it that) assumes that you

will be using the aps-c lens in the same way as its

full frame counter part.

 

meaning if you use 50mm at f/1.8 lens on a full frame

camera like a Sony a7 for a closeup, and a cropped 50mm

at f/1.8 on Soy a6000 also for a closeup, meaning both

cameras framed on the same subject, in the same way,

then the math holds. that's the laws of physics.

 

that's the way you would expect an average person to

use these lenses though.

 

.............

Explained as above, I fully agree with chrisqphoto ....

despite my long post to the opposite position.

another thing to keep in mind, is that an f/stop is not a measure of how much light reaches the sensor. that's a t/stop (transmission stop) which is totally different and measured a different way. it's more commonly used in cinematography, but be aware that it's very possible for a f/2.8 lens to actually have a t/stop rating of T/3.5.

so the actual amount of light you get, if you really want to measure it, get ready to pull out your calculators and consult with DXOMark.com LOL

As worded above, I AGREE with what chrisqphoto says,

despite my long post to the opposite position.

 

 

The reason for AGREEMENT is where chrisq says:

 

 

 

........ you use 50mm at f/1.8 lens on a full frame

camera like a Sony a7 for a closeup, and a cropped 50mm

at f/1.8 on Soy a6000 also for a closeup, meaning both

cameras framed on the same subject, in the same way ....

 

He's right about that. If you frame the subject the same,

then the 50mm on APSC requires your to increase subject

distance by 50%, which will also increase DOF if it's

easier to explain that way for some folks [everybody has

struggled with the loss of DOF in close focus shots].

 

The reason for my generalized disagreement with chrisq is

related to his statement:

 

 

that's the way you would expect an average person to

use these lenses though.

 

 

I just don't buy into that. On the FF a 50mm is a normal

lens, on an APSC it's a short tele. We don't choose a tele,

even a short tele, to step back so as to get the same shot

as we get from a normal lens. We use a tele to reach out,

to get a tighter framing even tho we physically can't step

closer to a subject. Thaz the general use of longer lenses.

 

OK. Here's the big hole in what I said immediately above:

 

IF it's a SHORT tele ...... and IF it's a PORTRAIT, thaz

where the I left a big hole. We frequently use a short

tele for the same framing we could manage of the same

portrait subject using a normal lens, but we want the

more flattering perspective that results from stepping

back to manage that framing.

 

So when chrisq says "that's the way you would expect an

average person to use these lenses though" it's kinda

like saying "the average person's usual application for

mild tele lenses is in portraiture". In that case he is

quite correct. His DOF explanation holds true for that

special case, for portraiture. For most other general

applications, the opposite explanation holds. This is a

fine example of the "special case" being the "most usual

case" for the "average person". IOW the "general rule",

that I bothered to explain at great length, really is an

accurate general rule when we consider all the wide

array of subjects and contexts for photography

 

But ! ! !

A "general rule" is bound to have exceptions, and in the

context of the "average person", a clear exception, the

making of pleasing portraits, is an extreeeemely popular

use of photography.

 

No wonder so many users complain about confusing advice !

 

The problem is that most advice, even correct advice, is

not carefully delivered complete with all the necessary

qualifying explanations. Advice tends to be given as the

immediate solution or guide to particular situation that

generates the OP of a thread. So far so good, for any

particular the OP.

 

But trying to more globally re-apply such advices DOES

cause confusion for readers of multiple "Help me" threads.

Physics is physics. Physical laws are immutable, and yet

the DOF rule for swapping format sizes while using one

single lens-on-hand is NOT the same for landscapes as

for portraits. The difference is between a "rule" and a "law".

DOF follows an immutable law. Varied applications of that

law will result in a variety of rules pertaining to a variety

of applications. ENJOY !

And I thought I was asking a "yes" or "no" question! Silly me!

 

But boy I sure have enjoyed this thread! It does respond to what I asked, and it also gives me lots to think about. I appreciate the education.

Explanations are really good here.

 

You can also start with a totally different aporoach as well. Take the rule of thumb, that for any kind of portraiture f/4 (FF) can be considered roughly adequate. Just apply the crop factor (e.g. on APS-C use f/2.8, on MFT use f/2) and start shooting. Use an adequate lens (ca. 85 mm on FF and shorter on smaller sensors). Play around and watch what happens. Change aperture, crop, whatever. Evaluate results.

Have fun!

 

Greetz, svenski.

Explanations are really good here.

 

You can also start with a totally different aporoach as well.

Take the rule of thumb, that for any kind of portraiture f/4 (FF)

can be considered roughly adequate. Just apply the crop factor

(e.g. on APS-C use f/2.8, on MFT use f/2) and start shooting.

Use an adequate lens (ca. 85 mm on FF and shorter on smaller

sensors). Play around and watch what happens. Change

aperture, crop, whatever. Evaluate results.

Have fun!

 

Greetz, svenski.

Excellent. My only critique of your helpful post is that it's

SO DAMNT SHORT ! ! ! ! !

 

For the humor impaired, just consider the source :-)

Also, my lower end zoom is the Zeiss f4 16-70. Does my thinking make sense in wanting this faster f1.8 lens?

 

Did anyone really address this part of your question?

 

Wanting the faster lens makes plenty of sense to me, for shutter speed reasons. It can make hand-held photography doable in some lower-light situations that the f/4 lens would not allow....or when the subject has a hard time sitting still.

 

I personally prefer a wider lens for indoor use, 16-19mm APS-C (24-28mm film equivalent).  But I like 30-35mm for general outdoor use like car shows or street scenes...that focal length at f/1.8 would be awesome.  

 

So I think you're on-target with the aperture, and the focal length depends on what you want to do with it. Having a few fast primes would be a nice complement to your f/4 zooms, IMO. But it's easy for me to spend your money. 

Did anyone really address this part of your question?

 

Wanting the faster lens makes plenty of sense to me, for shutter speed reasons. It can make hand-held photography doable in some lower-light situations that the f/4 lens would not allow....or when the subject has a hard time sitting still.

 

I personally prefer a wider lens for indoor use, 16-19mm APS-C (24-28mm film equivalent). But I like 30-35mm for general outdoor use like car shows or street scenes...that focal length at f/1.8 would be awesome.

 

So I think you're on-target with the aperture, and the focal length depends on what you want to do with it. Having a few fast primes would be a nice complement to your f/4 zooms, IMO. But it's easy for me to spend your money.

you are right, I missed that part of Lioneye's question.

 

basically, if you follow my equation provided, assuming you have read it and my explanation above, an F/4 lens on aps-c will provide roughly the same exposure as F/4 lens on a full frame camera, but in the depth of field category, it will behave like an F/6-ish (roughly 1.53x crop factor). so anywhere you can, if F/stop matter you and you want as much light as possible and the shallowest depth of field possible on an aps-c camera, get lenses that are at least 1 stop faster than you need.

a great tool to test my math out and see for yourself how multiple lenses compare on different camera bodies is to go to www.dofsimulator.net

 

very useful. will help you do all this math much faster than with just a calculator

OK here's another factor thaz based more on the 

"reality sometimes sukks" philosophy and less on 

the immutable laws of physics. 

   

If you're gonna shoot with at at least a modest dose 

of DOF, then you can choose a lens that only opens 

to the widest stop you intend to use. Frinstance, it's 

very OK [in formats up to 24x36mm] to focus at f/4.5 

when you intend to shoot at f/4.5. So an f/4.5 or f/4 

lens is no problem. 

   

But if you actually work at f/2 quite often, you want a 

lens faster than f/2 or f/1.8 [both of those being more 

or less the same]. Any slight error in focus, whether 

MF or AF [errors occur in both modes] will appear 

far more serious in the realm of really minimal DOF 

than it would have appeared in the realm of f/4.5 or 

f/4. Sooooo .... it makes sense to focus at least one 

stop wider than you shoot, to provide a safety margin 

against focus error. 

   

If you use an f/4 lens, and shoot at f/4 or f/4.5, then 

when you switch to an f/1.8 lens [similar FL] you'd be 

better off to not shoot any wider than f/2.5 [assuming 

similar subjects as the f/4.5 scenario, but needing a 

wider stop due to lack of light or to soften focus of a

distracting background].   

   

IOW, the nice clean math formulae for applying the 

laws of physics do fail to consider that applications of  

those laws will usually occur in real-world, imperfect, 

error-prone situations. Thus the Rule of Law must be

tempered by the Rule of Thumb :-) Frinstintz, pianos 

are tuned in reasonable compliance with the laws of 

physics, but definitely not to the "letter of the law", as 

the latter would sound weird/out/wrong to the ears of 

real world listeners. The piano analogy is not about 

error and safety margin, but it still demonstrates how 

Rule of Thumb tempers Rule of Law.

...it still demonstrates how

Rule of Thumb tempers Rule of Law.

true, but I believe your post assumes that the shooter is using the camera for video purposes. Video is a completely different animal of course and you are correct, in video maintaining focus is far more difficult. even with years of practice this is difficult. so in that I agree.

meanwhile in photography, this depends on the style of shooting, as some really like that style of ludicrous shallow DOF. there is a subject element here. but it also depends on the style of video one may be making.

 

while to some it is nauseating to watch a shallow DOF video, this style of shooting is in vogue and even high demand in circles. so I will revise your equation to say:

 

(Rule of Law) - (Rule of Thumb) + (Personal Tastes) = Finished Result

Every day is illuminating here on the forum! So my a6300 and Sony 70-200 f/4.0 G arrived and I went to the zoo to test it out. If this post works, here are three examples of shallow DOF. 

 

 

 

Hi -

 

I’m thinking about getting a fast prime to go with my zooms which are fixed at f4. I’m thinking of the Sony SEL 35 mm f1.8. I realize that on my (ordered) a6300 that the 35mm will be equivalent to about 52mm in full frame terms, but… does the f1.8 have an equivalency also? Does it convert to like a f2.8 or does it stay f1.8 because the ratio to the lens length is the same? (“IF” I understood that correctly?)

 

Also, my lower end zoom is the Zeiss f4 16-70. Does my thinking make sense in wanting this faster f1.8 lens?

 

Thanks All

 

Does the f1.8 have an equivalency also? => yes, in terms of DoF and total light

 

Does it convert to like a f2.8 or does it stay f1.8 because the ratio to the lens length is the same? => provide recomposing, it's most similar to f/2.8 in terms of DoF and total amount of light (although not quite as far a light goes, buy closer)

 

Does my thinking make sense in wanting this faster f1.8 lens? => Yes, faster lenses are always good in low light situations