[bigdavephoto]

Ok, I am going to ask what is, by my account, one of the dumbest questions I have ever asked considering that I have been taking photos for 30+ years.


If you have two lenses, lets say a 50mm f1.7 and a 28-80 f4-5.6, if you set the zoom lens to 50mm and both lenses to f 4, and your subject is 10 feet away, will the actual depth of field be the same in both lenses?


Just something I have been thinking about for a little while because DoF and aperture is the one area I sometimes have a problem wrapping my head around.

[fb_penpho]

Yes - they are the same.

A Flexible Depth of Field Calculator try this to check.

[stein]

I was just tinkering the same a few days ago, just didn't have the nerve to ask a question that I thought that I might be ridiculed for... Except I would put DA 50mm f1.8 vs the kit 18-55mm DAL in the equation... Thanks for asking instead of me

[bigdavephoto]

I'll take a look. Thanks.

---------- Post added 04-06-16 at 02:29 PM ----------

I was just tinkering the same a few days ago, just didn't have the nerve to ask a question that I thought that I might be ridiculed for... Except I would put DA 50mm f1.8 vs the kit 18-55mm DAL in the equation... Thanks for asking instead of me

That's part of the reason I was asking. I just got a DA 50mm and started to wonder if the DoF would be the same as a 28-80 I have.

[UncleVanya]

The real truth is slightly more complex. The fact is that focus breathing, manufacturer hype, lens design, etc. could cause small differences in the actual focal length of each lens. It would also be possible that the labeled f/stop might not be completely accurate. But in theory those minor details set aside - yes the same focal length and same aperture used on the same sensor size = the same depth of field.

[stevebrot]

will the actual depth of field be the same in both lenses?

Should be if FOV, crop, and final display size is the same.


Steve

[stein]

Tried also with the 90mm (macro) to see how far the bug actually has to be to get DOF that could give me a sharp image of the whole thing without need to do stacking. Well, pretty far for only a few millimeters at f8 and only one third of that at f2.8 So, need to start stopping down much more often...

[RKKS08]

The depth of field will be the same (defined as the range where a point of the object will form a circle on the film/sensor not exceeding a certain diameter).

But how such "points" slightly outside this range will look, will be different for every lens design (optics and sometimes also shape of the aperture).
This will influence what nowadays is described as "bokeh".

So the sharp area may look the same, the picture as a whole probably not. Even if the two lenses offer the same resolution, contrast, and colour rendering.

[Na Horuk]

To condense:
- focal length
- focus distance
- aperture
These are the factors that affect how the lens render's DoF. If you use three different lenses, but the three factors listed above are the same, then the DoF will be practically the same.
Then there are camera factors. First is sensor size. If you use same lens, focusing, aperture, but different sensor size (like FF, APSC, Q, 645), the whole photo will appear to be different because the sensor will capture more or less edges. But this does not actually change the DoF - DoF will only change once you start changing the above mentioned factors (for example, if you try to keep the framing the same, but use different sensor sizes). Things like a higher MP count and pixel peeping can also make DoF "appear" to be different, because you will be more critical of where it starts and ends, than with a camera with lower MP, stronger AA filter, or softer lens. This is because DoF is not just a purely scientific thing - it depends to an extent on perception.

However, even if the three factors I listed above are the same, two lenses labelled as 50mm f2 can give you different overall picture. Same general DoF, but one might have more detail within that DoF (sharpness), one might have different-looking OoF areas/bokeh, starbursts, different distortion, coma, CA, even colours and contrasts.

Anyway, DA 50mm f1.8 is a great lens, will give you a lot of advantages over the 18-55mm, especially in lower light conditions, or when you want critical sharpness.

[672]

if you set that zoom lens to 50mm you will not get a 4 stop. maybe a 4.5 , more likely a 5.0 .
also, a slow zoom lens like that is generally going to be made of cheaper glass than the prime lens. the prime should appear to have less depth because the in focus bits will be sharper. the zoom lens will have a mushier look , but with more apparent depth.