Calculating DoF...

I'm trying to calculate how the DoF will compare with different focal lengths. I'm using this page: http://www.dofmaster.c​om/dofjs.html

The problem is that I don't know how to calculate the distance. For example, if I'm using an 85mm lens at 10ft from the subject, how far from the subject do I have to stand to have the subject fill the same amount of the frame - with 200mm?

I thought I just divided 200 by 85 (2.35), and used that as the multiplier on my base distance (10). But, I doubt my results with that method.

icor1031 wrote in post #18780030
I'm trying to calculate how the DoF will compare with different focal lengths. I'm using this page: http://www.dofmaster.c​om/dofjs.html

The problem is that I don't know how to calculate the distance. For example, if I'm using an 85mm lens at 10ft from the subject, how far from the subject do I have to stand to have the subject fill the same amount of the frame - with 200mm?

I thought I just divided 200 by 85 (2.35), and used that as the multiplier on my base distance (10). But, I doubt my results with that method.

I suppose you want to know FoV (field of view) match and not depth of field (focused/bokeh) match. There is also a camera sensor (crop) factor to be considered:

https://photography-on-the.net/forum/fov_calc​ulator.php

Full frame:
85mm FoV = 23.9 degrees
200mm FoV = 10.3 degrees

I bet someone can show a formula you need from there, I suck with math

icor1031 wrote in post #18780030
I'm trying to calculate how the DoF will compare with different focal lengths. I'm using this page: http://www.dofmaster.c​om/dofjs.html

The problem is that I don't know how to calculate the distance. For example, if I'm using an 85mm lens at 10ft from the subject, how far from the subject do I have to stand to have the subject fill the same amount of the frame - with 200mm?

I thought I just divided 200 by 85 (2.35), and used that as the multiplier on my base distance (10). But, I doubt my results with that method.

That is correct.

john crossley wrote in post #18780062
That is correct.

When I did that, the DoF matched. That is, f/1.4 always gave .35ft DoF - regardless of FL.
I thought higher FL was supposed to give less DoF? Are people just ... sillly, when they talk about how amazing and shallow the 200/2 is?

icor1031 wrote in post #18780170
When I did that, the DoF matched. That is, f/1.4 always gave .35ft DoF - regardless of FL.
I thought higher FL was supposed to give less DoF? Are people just ... sillly, when they talk about how amazing and shallow the 200/2 is?

You discovered the REALITY of DOF calculations! Congratulations for the astute observation that so many never make!
...when the subject is the IDENTICAL SIZE within the frame (that is, it occupies the same percentage of the frame regardless of FL used) the DOF is the same!

...the 'longer FL gives shallower DOF' only applies when the object size is larger within the frame compared to using the shorter FL.
When the object size is identical, FL does not matter!!

Wilt wrote in post #18780203
You discovered the REALITY of DOF calculations!
...when the subject is the IDENTICAL SIZE within the frame (that is, it occupies the same percentage of the frame regardless of FL used) the DOF is the same!

...the 'longer FL gives shallower DOF' only applies when the object size is larger within the frame compared to using the shorter FL.
When the object size is identical, FL does not matter!!

So, those 200mm guys are silly ... alright.

I wish medium format cameras were made in smaller packages!

icor1031 wrote in post #18780208
So, those 200mm guys are silly ... alright.

I wish medium format cameras were made in smaller packages!

You hear about fashion photographers working with models and 400mm lenses, and with the insight that 'same size in frame' is 'same DOF' it might seem silly to have to communicate with your model by shouting loudly to be heard when you do not need to do so if you use 200mm or 100mm and modify your shooting distance to suit.

But one cannot forget the fact that although the DOF is 'the same', the longer FL definitely has the advantage of presenting the farfield background much MORE BLURRY!

Wilt wrote in post #18780224
You hear about fashion photographers working with models and 400mm lenses, and with the insight that 'same size in frame' is 'same DOF' it might seem silly to have to communicate with your model by shouting loudly to be heard when you do not need to do so if you use 200mm or 100mm and modify your shooting distance to suit.

But one cannot forget the fact that although the DOF is 'the same', the longer FL definitely has the advantage of presenting the farfield background much MORE BLURRY!

A local photographer does that - he uses a long FL in studio, and one model that we both worked with commented to me how annoyed she was that she couldn't hear him.

Also, nice resource! (the site in the image)

icor1031 wrote in post #18780263
A local photographer does that - he uses a long FL in studio, and one model that we both worked with commented to me how annoyed she was that she couldn't hear him.

So, the higher FL does produce more blur; it transitions to OOF more quickly(?) - and essentially, in basically the same way that full frame would?
Could you show me how 85/1.4 would compare to 200/2?

Thank you!

On a single body, in order to frame the subject the same with 85 vs 200, you have no choice but to change your distance. This changes the DOF and also the perspective of the subject to their surroundings. FL changes framing, distance changes framing, both affect DOF, along with aperture.

icor1031 wrote in post #18780208
So, those 200mm guys are silly ... alright.

Not sure what you are trying to say, but there are very good reasons to use long lenses. Narrow angle of view compresses the background which often reduces distracting elements in the photo. It also helps keep objects nearer the camera from appearing larger in relation to other objects

Left Handed Brisket wrote in post #18780326
Not sure what you are trying to say, but there are very good reasons to use long lenses. Narrow angle of view compresses the background which often reduces distracting elements in the photo. It also helps keep objects nearer the camera from appearing larger in relation to other objects

"Those" refers to the 200mm users who make a certain claim, as I mentioned in my prior(?) post. Also, as Wilt showed - they're not necessarily wrong.
I know it changes FoV and perspective distortion, but that's not my focus.

Wilt wrote in post #18780224
But one cannot forget the fact that although the DOF is 'the same', the longer FL definitely has the advantage of presenting the farfield background much MORE BLURRY!

It needs also to be said that distance affects perspective, if you shoot from 15m with a fisheye, the perspective is same as it would be with 1200mm. This in turn affects composition, which means all these factors contribute to the "look" of the image, if you change your distance when changing focal lengths.

https://en.wikipedia.o​rg/wiki/Perspective_(g​raphical)

Ttell her to tell him, "Quit cheaping out and buy & use an FRS transmitter and receiver."

Pekka wrote in post #18780357
It needs also to be said that distance affects perspective, if you shoot from 15m with a fisheye, the perspective is same as it would be with 1200mm. This in turn affects composition, which means all these factors contribute to the "look" of the image, if you change your distance when changing focal lengths.

https://en.wikipedia.o​rg/wiki/Perspective_(g​raphical)

Agree completely. But many fail to understand that amount of background blur is somewhat disjoint from DOF depth, which is why I mntioned only background blur relative to DOF

Wilt wrote in post #18780573
Agree completely. But many fail to understand that amount of background blur is somewhat disjoint from DOF depth, which is why I mntioned only background blur relative to DOF

I played with the site that you got your graph from for a while... It looks to me that going medium format would actually be a disadvantage, compared to full frame - at least for outdoor shoots. We have a 50mp FF sensor, and lenses that can provide 40+ mp. So, I don't think going MF for more detail is reasonable. Regarding blur, f/1.4 FF is exactly equal to f/1.75 MF. So, you're losing light (t/ will be higher) - but realistically, what are you gaining?

The same isn't true when comparing crop to FF. It was about f/0.87 vs f/1.4. There are no good lenses (afaik) with an f/ that low. But if they existed, sure - crop would be better (again: more light.)

Does it seem that I've understood correctly?