FF vs. Aps-c DOF/Bokeh

[GeneralBenson]

Ok, so I have a question about the differences of the depth of field and bokeh on full frame versus aps-c sensor cameras. I used to think that it was just effective focal length that determined the DOF. So if full frame is at 200mm, and aps-c is at 135mm, then bokeh would be the same. I know know that's not true.

So what I think I know, and may or may not be right, is that bokeh is only determined by the distance of the rear element to the image sensor, i.e. actualy focal length. So FF at 200mm would have the same DOF as aps-c at 200mm, despitethe fact the the effective focal lengths would be different, right? So FF cameras will always get a shallower DOF at a given focal length, right?

So again, that wha I think I know, but a view days ago I was out shooting with a friend. He was using a 1Ds MkIII and 70-200L, I was using a Pentax K20d and DA* 50-135 f/2.8. So we're using the same focal length equivelent, but of course getting different DOF. But what tripped me up was when we were shooting the same subject, on the same background, from the same distance, and he was getting less DOF despite the fact that he was at a shorter focal length. I was shooting around 100mm, so effectively 150mm, and he was shooting at 75mm, which is still 75mm. But he was clearing have a shallower DOF. What the deal with that? I'm pretty sure we were both shooting at f/2.8.

 

[Big Mike]

I think you are confused about 'effective focal length' and the whole crop factor issue. When using a camera with a smaller sensor (than the standard 35mm format)...the only thing that changes, in terms of focal length, is the field of view you would expect. The perspective etc does not change. It's the same image, only cropped.

Now, in terms of DOF, we need to consider than a smaller capture area gives you more (deeper) DOF. So your friend's camera, which is 'full frame' will naturally have a shallower DOF that your camera and the focal length difference isn't enough to make up for it.

There are advantages and disadvantages to this. For example, those who want a shallow DOF, it's easier to do with a full frame DSLR. On the other hand, if you need more DOF and are limited by light levels, you can get more DOF on a crop DSLR, in the same light.
I know a pro wedding photographer who shoots with several cameras, including the 1Ds mkIII, 1D mkII, 40D, 20D etc. He likes the 40D for shooting at the ceremony because it's easier to get two people in focus while shooting at F2.8, than it is with the bigger sensor cameras.

 

[Garbz]

Depth of field is a function of subject distance to camera. The focal length of the lens ultimately only determins the magnification of the subject. Now the idea is that you have a 200mm lens, it appears longer on a camera with a crop factor, thus when the lens is set to 200mm the crop camera needs to be further away from the subject increasing the camera to subject-background ratio and thus reducing the depth of field.

Conversely if two people are standing the same distance of the subject, because of crop factor the full frame body can be at 200mm, but the cropped body would need to be set to 150mm to capture the subject in the same size and the same depth of field. Thus the full frame camera at a given focal length will have a greater depth of field due to the greater background-subject compared to subject camera ratio.

Sorry if this is confusing it was hard enough writing this without writing it properly :S.

 

[Sark]

Sorry if this is confusing it was hard enough writing this without writing it properly :S.

Hmmm..a bit confusing, yes.

Basically, three things affect DOF. Focal Length, Lens to subject distance & the F number.

Cropped sensors require shorter lenses to match the same field of view as full frame sensors, so for the same field of view the required shorter lens on a cropped sensor produces a greater DOF.
Alternatively, with the same lens attached to both a cropped and full frame sensor, the cropped sensor would need to increase its lens to subject distance to match the same field of the view as the full frame sensor. The change in lens to subject distance again increasing DOF.

Sark

 

[inTempus]

Here's a visual:

The largest box (black frame) represents full frame. The others represent 1.3 (Canon), 1.5 (Nikon) and 1.6 (Canon).

 

[Seefutlung]

It is my simple understanding that sensor size has a direct relationship with DOF. Using same aperture and same focal length you'll get less DOF with a FF than an APS-C sensor and an APS-C sensor will have less than a 4/3rd's sensor and a 4/3rd's sensor will have less than a P&S sensor.

Gary

 

[epp_b]

^ Correct.

 

[JerryPH]

Using same aperture and same focal length you'll get less DOF with a FF than an APS-C sensor

This is one of the main reasons I *luv* my D700, I am constantly impressed with how much less DOF we have at similar apertures, and how much simpler it seems to be to get that look and feel that I want from my D700 than from the D200 under the same circumstances.

 

[epp_b]

Ah ... one of these days, the curves of "cheapest full-frame camera" and "I can afford a full-frame" will intersect

 

[Seefutlung]

Ah ... one of these days, the curves of "cheapest full-frame camera" and "I can afford a full-frame" will intersect

All a matter of priorities ... I am forgoing food for the next year ana half in order to pay for my FF's.

Gary

 

[Garbz]

Yeah and with the dramatically reduced muscle content in your body as you starve how will you find the energy to hold the typically much heavier full frame cameras

 

[Seefutlung]

Yeah and with the dramatically reduced muscle content in your body as you starve how will you find the energy to hold the typically much heavier full frame cameras

Damn ... the devil is in the details.

G