Clearing up a focal length question with aps-c lenses

[orobas]

Ok.. so i have a A6400 which has an APS-C sensor..

I use a range of lenses of which include both full frame and crop sensor designed lenses..

I know using a full frame lens i get a crop zoom of 1.5x .. so for example my Sony FE 50mm becomes a 75mm, my Tamron 70-300mm f4.5-6.3 DI III RXD becomes a 105-450mm

Now.. if i use a aps-c lens.. i get the exact mm yes ? ie.. if i use the Sigma 56mm f1.4 DC DN .. this IS a 56mm on my A6400 as the DC denotes the lens was made for APS-C sensors and it even has the aps-c cut out on the back of the lens.

Do i read this right ? .. or do i still have to put a x1.5 35mm zoom in effect.. so my 56 becomes an 84?

This is something that has always confused me...

Can someone clarify just what mm i am getting using a dedicated aps-c designed lens on a aps-c sensor please!

[Pieter]

Crop factor is a property of your camera(sensor), not of the lens. Focal length is a property of the lens and has nothing to do with the camera. The only difference between a lens designed for APS-C and a lens designed for fullframe is that a fullframe lens can cover a larger image circle if used on a bigger sensor. On your A6400, a 56mm lens will always look the same, be it designed for APS-C or fullframe.

[orobas]

Thanks Pieter..

I've had a guy going crazy on me saying regardless of the lens i am using on my aps-c A6400 i need to multiply it by 1.5 to get the correct focal length and that my 56mm is a 84... my 16mm is a 24mm not a 16mm and it's left me totally confused

[Pieter]

The guy you spoke to was wrong and is spreading the confusion. It's not like the fullframe equivalent focal length is the 'true' focal length of a lens. Your 56mm is as true a 56mm lens as if it were designed for fullframe, medium format or micro 4/3. Crop factor is only relevant when comparing one system to another. If all you've ever used is an APS-C system and know what a 56mm lens looks like on your camera, why would you want to relate to a fullframe system? Somehow mankind decided fullframe is the general standard to relate to when talking about equivalent focal length. Could have been any other format.

[orobas]

Many thanks

So... moving on a sec.. bear with me

I have a couple of full frame lenses that are same mm as my aps-c lenses but i notice the difference between the 2 when i take the same photo. (the full frame lens has a full circle at the back where the aps-c has the rectangular cut out at the back of the lens) .. I have to step a lot further back with a full frame lens than i do with a dedicated aps-c lens to get the same framing

That's what led me to understand that if i was using a dedicated full frame lens on my aps-c sensor i need to apply the zoom factor of 1.5x due to the focusing of the lens on the smaller sensor.

Did i get that right?

 

[Pieter]

It is common that the focal length mentioned on your lens is not exactly what it claims to be. A 50 mm lens can really be a 46 mm or 54 mm lens. This might cause a difference in field of view between any two '50 mm' lenses. But it surprises me there is a couple of steps difference between the two to get the same framing. Care to share a pic of the two lenses with focal length markings?

[orobas]

Both 30mm.. One is the Sigma 30mm f2.8 DN, the other is the Sony 30mm F2.8 SAM (aps-c)with adapter. Both taken at the exact same spot. 

The one with the Rowing Boat was f/11, 1/50 and ISO-320. 

The one without was f/8, 1/50 and ISO-160. 

taken from exif

 

 

 

Welcome, dear visitor! As registered member you'd see an image here…

Simply register for free here – We are always happy to welcome new members!

Edited May 16, 2022 by orobas
will only let me upload one imade

[Pieter]

These seem pretty identical to me, except the top photo is taken slightly more to the right and the bottom photo is taken a bit more to the left.

The Sigma 30mm f/2.8 DN is also designed for APS-C by the way.

[orobas]

oh gawd i didnt notice i'd possibly knocked the tripod on lens change... I feel like a right idiot now lol..

I'll have to try and replicate this again as soon as i can from a static point where camera body is clamped down.

I thought the sigma 30mm f2.8 was full framed as it was a full circular cut out at the rear (had no rectangular aps-c back) and the lack of the DC (for apc-s) marking.. My Sigma on closer look is the DN A version... with the tiny front glass 

Welcome, dear visitor! As registered member you'd see an image here…

Simply register for free here – We are always happy to welcome new members!

[Pieter]

Yes, that one is for APS-C. Guess it was before Sigma started using the DC designation to specify an APS-C design.

 

[orobas]

how odd... only other thing i can think of is the field of view angle then between the lenses.. as to why i have to alter positions 

[XKAES]

4 hours ago, orobas said:

Thanks Pieter..

I've had a guy going crazy on me saying regardless of the lens i am using on my aps-c A6400 i need to multiply it by 1.5 to get the correct focal length and that my 56mm is a 84... my 16mm is a 24mm not a 16mm and it's left me totally confused

 

That guy is kinda correct, but I agree with Pieter -- the way he is presenting it can be confusing.  As Pieter suggests you need to think APS format when thinking lenses -- not FULL-FRAME format.  The exact same thing applies to 35mm full-frame cameras and half frame cameras.

A 50mm lens is consider a normal lens for a full-frame digital or film camera.  If you put the same lens on an APS or half-frame camera it is still a 50mm lens, but the sensor only sees the central section -- so what you get is a cropped image, or longer focal length effect.  You would get the same effect if you used a 75mm lens on a full frame camera.

So if you want the same normal lens effect of a full-frame 50mm lens on an APS or half-frame camera, you need to use a shorter focal length lens -- such as 30mm.

It's easier if you think APS format when thinking of focal lengths, and not FULL-FRAME format -- although people who use both APS and full-frame cameras get used to it.  It sort of like speaking French and Spanish -- similar in some ways, but not the same.

 

[orobas]

@XKAES

ah..... i think i have it now..

So my Tamron 70-300mm f4.5-6.3 DI III RXD full frame lens on my APSC-C sensor....

It is still 70-300 on my aps-c regardless.. but because it has a cropped sensor it gives an equivalent photo to as if i was using a 105-450 full frame lens on a full frame sensor due to the field of view comparison?

So that 1.5x is a field of view .. not zoom... but if i wanted to get that same field of view on a full frame camera body.. then i would need a lens 1.5x the zoom (provided the full frame camera did not have a crop mode button)

And that 1.5x cropped field of view looks bigger because the sensor covers that narrower area more than what a full frame sensor does so it gives you that zoomed in effect 

gotcha! Your explanation has really helped me understand it!

so that is what the 35mm focal length is then on the exif that i see...  "if you wanted to see this exact same photo image with all full frame equipment.. you need a lens with 1.5 times the zoom"

[Pieter]

4 hours ago, orobas said:

@XKAES

So my Tamron 70-300mm f4.5-6.3 DI III RXD full frame lens on my APSC-C sensor....

It is still 70-300 on my aps-c regardless.. but because it has a cropped sensor it gives an equivalent photo to as if i was using a 105-450 full frame lens on a full frame sensor due to the field of view comparison?

Exactly! That is the perfect way of summarizing it.