[k100d]

one image the OOF is much blurrier than the other, i got it right :P
nice test steve

[axl]

I'd say the top is 85 and bottom 50. Judging purely by perspective...
... in the top picture the relation between the two flowers in the foreground and the one in the background is more compressed (flatter) which makes it longer focal lenght, the bottom shot has greater sense of distance between foreground and background which would suggest shorter lens...
BR

[Vormulac]

I would say the top one is the 85mm, as the background is more out of focus and DoF is associated with (among other factors) focal length.
But I'm a beginner, what do I know?

[Wheatfield]

I would say the top one is the 85mm, as the background is more out of focus and DoF is associated with (among other factors) focal length.
But I'm a beginner, what do I know?

Apparently you don't know that depth of field isn't related to focal length......

[wlank]

Isn't there a forum around here somewhere for lens discussion?

[attack11]

But let's make it clear that the DoF is exactly the same for both of them.

how do you know that when he doesn't say the distances to the subject so you can do an actual calculation? even if the dof is the same, the magnification and perspective is going to be different.

[attack11]

this thread hurts my head. focal length is used to determine a general area for the dof in calculations, but every lens model has it's own magnification which will affect the persective, distortion, etc.

this isn't even a valid test because it's different focal lengths, all you did was find generally a sweet spot of distance where the result looks similiar if you don't look too close.

fov crop does not turn a 50mm lens into a 75mm. it's still a 50mm lens, but what you see from it is similiar to the fov of a 75mm lens. you lose a lot of the border of the picture.

[Blue]

But let's make it clear that the DoF is exactly the same for both of them.

Then why does the math come out differently when calculating depth of field for a 50mm macro lens than a 105mm one? I'd go dig out my freshmen physics book (2nd semester is when this stuff was covered), but I burned it.

[stevebrot]

Isn't there a forum around here somewhere for lens discussion?

Yes, there is...The thread was placed here because this is a general photography matter. I can remember this issue being a matter of discussion 40 years ago when I first started taking pictures as it relates to choice of lens for a particular subject.

Steve

[stevebrot]

how do you know that when he doesn't say the distances to the subject so you can do an actual calculation? even if the dof is the same, the magnification and perspective is going to be different.

I thought it was clear in the original post, but here it is again:

1) Two different focal lengths

2) Same aperture (f/5.6)

3) Different camera distance

3) Neither picture cropped (therefore same magnification)

Remember, magnification is the size of the subject on the sensor/film relative to the actual size.

I left it up to the viewer as to whether the DoF is equivalent. Is there anything in the first shot that is in/out of focus that is not in the second?

Note that the while the out-of-focus rendering and the relative size of the out-of-focus objects are quite different, that is not the same as DoF.

[stevebrot]

this isn't even a valid test because it's different focal lengths, all you did was find generally a sweet spot of distance where the result looks similiar if you don't look too close.

The test was to demonstrate that different focal lengths will provide equivalent DoF for the same magnification and aperture. (I.e. DoF is a factor of aperture and magnification, not focal length.)

So...explain to me again how the setup does not satisfy the condition? On the other hand, perhaps you could continue the thread with a setup of your own design?

Steve

[attack11]

I thought it was clear in the original post, but here it is again:

1) Two different focal lengths

2) Same aperture (f/5.6)

3) Different camera distance

3) Neither picture cropped (therefore same magnification)

Remember, magnification is the size of the subject on the sensor/film relative to the actual size.

I left it up to the viewer as to whether the DoF is equivalent. Is there anything in the first shot that is in/out of focus that is not in the second?

Note that the while the out-of-focus rendering and the relative size of the out-of-focus objects are quite different, that is not the same as DoF.

well, see the issue is you use the term equivalent. on technicality alone, they're not equivalent. they just look "close".

[attack11]

I thought it was clear in the original post, but here it is again:

1) Two different focal lengths

2) Same aperture (f/5.6)

3) Different camera distance

3) Neither picture cropped (therefore same magnification)

Remember, magnification is the size of the subject on the sensor/film relative to the actual size.

I left it up to the viewer as to whether the DoF is equivalent. Is there anything in the first shot that is in/out of focus that is not in the second?

Note that the while the out-of-focus rendering and the relative size of the out-of-focus objects are quite different, that is not the same as DoF.

well, see the issue is you use the term equivalent. on technicality alone, they're not equivalent. they just look "close".

reasons... magnification, dof, bokeh, distortion ... you just can't compare it. it's like doing aps-c to full frame dof comparisons with the same glass; it's not the same.

[stevebrot]

fov crop does not turn a 50mm lens into a 75mm. it's still a 50mm lens, but what you see from it is similiar to the fov of a 75mm lens. you lose a lot of the border of the picture.

You are so correct. The 1.5x crop factor gets you a narrower fov and the same perspective as a longer lens, but that is all. The set of pictures with the lens/cutting board (same distance, different focal length, different crop factor).

[stevebrot]

well, see the issue is you use the term equivalent. on technicality alone, they're not equivalent. they just look "close".

reasons... magnification, dof, bokeh, distortion ... you just can't compare it. it's like doing aps-c to full frame dof comparisons with the same glass; it's not the same.

Ummm...as suggested above...maybe you should do another test to prove your point. Instead of flowers, maybe you could use a low angle shot of a measuring stick and provide the camera to subject distances. I am interested in seeing your results. After all, the original post was just a quick/dirty demonstration. (Only took about 8 minutes)

Steve

BTW I used the term equivalent because it is a good waffle word and I did not provide anything in the picture that could be used for quantitative analysis. I am also curious...were you able to tell the two lenses apart based on DoF?