crop lens fitted to a A7

[adwb]

ok so I know that if you fit a full frame lens for example, a 50mm f2.8 to a 1.5 drop factor body,  the effective viewing size becomes equivalent to a 75mm and the f stop becomes an effective  f4 as far as dof goes, but what happens if you fit a e-mount crop frame lens to a A7 , in auto it also looks like a 75 mm but there also the same shift in DOF?

[Guest all8]

I think you will get a long argument on DOF.

 

The lens remains unchanged, the DOF shift is caused by the smaller sensor of the APS-C (and pixel size), and how much that smaller image must be magnified to produce an image of the same size as would come from a FF sensor - i.e. about 1.5 times more magnification. So if you use "crop mode" then yes it will have the same shift.

[adwb]

yes but my question is the reverse of the normal I am thinking to use a sigma art e-mount 60mm f2.8 which is designed for a crop frame and therefore  vignettes heavily [unless put on auto] on a full frame A7,

so it case what happens to the dof is it still 2.8 or is it also f4.2?

[Guest all8]

Well it can't be both!!!!! If you put it on auto it is 4.2, otherwise it depends how much you crop ....

 

It does not matter, these equivalent DOF calculations are a waste of time and have almost no relavance outside of archaic calculations. You will have to take photos and understand how the lens behaves.

 

If I tell you its 4 and you own experience tells you its 5 what are you going to do, argue with me or take photos?    

[Colorado]

OK, rather simple question to answer. If you are closer to the subject with the full frame camera to get the same image as you would on the crop sensor the DOF will be equivalent to any other 2.8 lens on the Full frame sensor. In other words, the distance to the subject is what gives you less DOF with a full frame compared to a crop sensor not the lens.

[duncanfoxphoto]

The short answer is the body type doesn't determine the depth of field, it determines the framing. A 50mm lens on a full frame body has the same depth of field as a 50mm lens on a crop body, except that the crop only shows the center part of the frame. "Effective focal length" is a bit of a misnomer, because it doesn't change the actual focal length like changing a lens would, but rather what length of lens you would need to use on a full frame body to get the same framing as on the crop frame body.

 

If you use a crop lens on a full frame body, you might as well just use the full frame lens and have room to crop in. It's a waste of a sensor.

[Golem]

The short answer is the body type doesn't determine the depth of field, it determines the framing. A 50mm lens on a full frame body has the same depth of field as a 50mm lens on a crop body, except that the crop only shows the center part of the frame. "Effective focal length" is a bit of a misnomer, because it doesn't change the actual focal length like changing a lens would, but rather what length of lens you would need to use on a full frame body to get the same framing as on the crop frame body.

 

If you use a crop lens on a full frame body, you might as well just use the full frame lens and have room to crop in. It's a waste of a sensor.

 

And if you do choose to crop [assuming a FF lens in use] your DoF deteriorates, cuz

essentially, you're taking a bigger, closer look at the image and its constituent details.

Cropping reduces DoF, so putting a FF lens on a crop body requires stopping down

a stop more compared to what a the DoF scale would advise, the scale on the lens

barrel being based on NOT cropping the image, meaning NOT putting the FF lens on

a crop body. I do agree it's a waste of a sensor.

  

DoF, judged by reasonably close inspection of the final image, is based on ONLY two

parameters. One is the final size of the image. The other is the f/stop. It doesn't matter

HOW the image gets to be that final size. You can crop a broad view, you can shoot a

tighter view. You can shoot the tighter view by approaching closer to the subject or by

using a longer lens. Cropping, using a longer FL, approaching close, all cost you DoF.

  

Strictly speaking, ALL images are cropped: cropped by the format edges, cropped by

using less than the full format, or cropped by the diameter of the outer lens elements

on both ends. For any FL, if the format size is unlimited, the only "optic" that doesn't

crop the image is a perfect pinhole. When you start using a bigger hole, and putting

glass optics on both sides of the hole, the size of the glass crops the pinhole's original

hugely wide view. All glass optics crop. Typically, we use a format that crops more of

the scene than the optics are cropping.

  

Acoarst that glass delivers benefits to compensate for imposing a crop. It evens out

the illumination, sharpens details, gathers huuuuugely more photons, etc etc. But it

also introduces chromatic aberrations, distortions, etc, which a pinhole does not. In

the grand scheme, the tradeoffs mean that most of us put up with the cropping that

glass imposes ... by using glass whose crop projects a larger scene than it takes to

fill the format size. A crop lens on a FF sensor defies this most reasonable practice.

[Golem]

OK, rather simple question to answer. If you are closer to the subject with

the full frame camera to get the same image as you would on the crop sensor

the DOF will be equivalent to any other 2.8 lens on the Full frame sensor. In

other words, the distance to the subject is what gives you less DOF with a full

frame compared to a crop sensor not the lens.

 

DoF won't change in your scenario where the image SIZE is constant.

However the framing will be tighter on the crop version. Your scenario,

as written, is ambiguously unclear. Distance ? Which distance ? 

[Golem]

ok so I know that if you fit a full frame lens for example, a 50mm f2.8 to a 1.5 drop

factor body,  the effective viewing size becomes equivalent to a 75mm and the f stop

becomes an effective  f4 as far as dof goes, but what happens if you fit a e-mount

crop frame lens to a A7 , in auto it also looks like a 75 mm but there also the same

shift in DOF?

  

If you actually put that lens on your camera, keep looking over your shoulder ......

[Colorado]

Golem, I was simply responding to the presented scenario. To better explain my point.:You have two 50mm 2.8  lenses, one full frame and one 1.5 crop and lets assume as stated in the question at hand, the crop lens, all be it poorly, will work on and cover the whole sensor on a full frame camera. Both lens will have the same DoF on the crop sensor camera and both will have the same shallower Dof on the full frame camera. The lens has no bearing on the difference in DoF between the crop and the full frame. If you have the same image rendered from both formats using all of both sensors and a 50mm 2.8 lens on each, the only difference needed to accomplish this is distance from sensor to point of focus. This is what gives you the difference the DoF.