Depth Of Field: Can it be increased with a cropped sensor?

Front to back  detail at F32 with 50 mm lens on a medium format camera

One particular kind of question I get asked a lot is in relation to "Depth Of Field" (DOF) and how it is different with a cropped sensor camera, especially when using wide-angle lenses. I have always been aware that there are changes, and that the DOF does increase in most cases. However, I have to admit that I haven't known the exact scientific reason why, until some personal research yielded some results which I wish to share with you.

My reason for revisiting this is that I have recently been using a "Fuji X Pro 1" much more, due in part to its portability, in addition to my love of "range-finder" style cameras. With this camera, I found that, in low light, even at at F1.4, I have a reasonable DOF with a 35 mm lens, even though as most of you will know, F1.4 should give a very shallow DOF. This type of camera has a crop factor of x 1.5 the sensor being smaller that a full sized 35mm sensor which the lens focal length are still measured by, meaning that a 35mm lens x 1.5 = 50 mm (actual focal length). Below in an illustration taken in the poorest of light conditions:

Almost front to back detail at F1.4 with a 35mm lens on a cropped sensor camera.

With the image above, the 35mm lens (effectively working as a 50 mm lens) still has the quality of a wider angle lens than a 50mm at F1.4 would normally have.

So, why is this?

There are four factors that effect DOF:

1. The F Stop, the larger the number (smaller the aperture) the greater the DOF will be.
2. The focal length of the lens the longer it is the small the DOF will be.
3. The distance to the subject, things further away near or at infinity will have a greater DOF.
4. The size of the film plane or sensor, so for instance medium formats 50mm equivalent is 80mm and for large format it is 110mm.
5. The quality of the lens, generally lens with low F Stop numbers especially prime lenses will have a the ability to have a good sharpness around the focal point either fully open or certainly when the F Stop is only slightly increased say from F.2 to F1.8,. also the sweet spot of the lens is also better utilised with cropped sensors as this area fills more of the frame.

It is important to say that this only applies to the normal use of lenses and in macro photography could not be relied on in the same way.

I hope that you have found this short blog article to be useful. Please come back here soon for more updates.