Why do depth-of-field calculators show more DOF for larger formats with the same focal length and f-stop?
Asked 3/25/2014
5 views
2 answers
0
When I compare depth-of-field calculators using the same focal length, aperture, and subject distance but different film/sensor sizes, the larger format sometimes shows more depth of field. For example: 100mm lens, f/8, subject at 20 feet. Some calculators show much more DOF on 4x5 than on 35mm, which seems opposite to my real-world experience that larger formats usually have shallower depth of field. What assumption am I missing? Is this about circle of confusion, enlargement, or comparing the wrong things?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
11
Here's what you're missing: that larger formats have less depth of field for the same framing, not at the same focal length. A 100mm lens is much wider on medium format than it is on 35mm film. If you keep that and the aperture constant, DoF will be identical assuming you print with the same enlargement (that is, the medium format print will be much larger). If you instead enlarge the 35mm print to the same size, you also enlarge the blur, yielding the decreased DoF the calculator is showing you. (It has these assumptions hard-coded into its chosen values for circle of confusion.)
Instead, adjust the focal length so that you have the same framing. If you hold that constant, you will find that your original understanding still holds: greater depth of field for the smaller format (even with greater enlargement). More background on this at previous question Can a smaller sensor's "crop factor" be used to calculate the exact increase in depth of field?
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
12y ago
0
Generated from our catalog & community — verify before relying on it.
The calculators aren’t really contradicting your experience—they’re using a different basis for comparison.
Depth of field depends on the acceptable circle of confusion, which is tied to how much the image must be enlarged for a final print/viewing size. A larger format needs less enlargement than 35mm to make the same print, so a larger blur spot on the film can still look acceptably sharp. That makes the calculator report more DOF for the larger format when focal length, f-stop, and subject distance are held constant.
But your practical experience is usually based on the same framing, not the same focal length. A 100mm lens on 4x5 gives a much wider view than a 100mm on 35mm. To match framing, the larger format uses a longer focal length or closer camera position, and that combination gives shallower depth of field. So:
- same focal length/aperture/distance + same output size: larger format can show more calculated DOF
- same framing/composition: smaller format gives more DOF
So the missing piece is that DOF is relative to circle of confusion and final enlargement, and format comparisons only match real-world intuition when framing is held constant.
Recommended products
UniqueBot
AI12y ago
Your Answer
Related Questions
Why don’t 25mm at f/4 and 100mm at f/16 give the same depth of field at the same subject distance?
Why do depth-of-field calculators change with sensor size, teleconverters, and focal reducers?
How do you calculate depth-of-field and bokeh equivalence between medium format and full frame?
Does background blur change with focal length when subject framing stays the same?
Does sensor pixel size affect depth of field or the circle of confusion?