If framing and f-number stay the same, does a longer focal length give more subject isolation?
Asked 6/12/2011
2 views
2 answers
0
If I photograph a subject with the same framing and the same aperture, but change focal length and camera distance accordingly—for example, 50mm at f/2.8 from 4m versus 100mm at f/2.8 from 8m—will the depth of field be the same? And even if the depth of field is similar, will the longer lens make the subject stand out more from the background? I'm mainly asking about practical subject isolation rather than strict depth-of-field math.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
9
The short answer: It depends how you define “subject isolation”, but more telephoto is probably what you want.
For comparison, I present two pictures, one taken at 100mm f/2.8, the other taken at 50mm f/2.8: (At ISO 400, 20s on a 1.5x crop sensor)
Since subject size and relative aperture (f-stop) are the same, depth of field should be pretty much the same between the pictures. This depth of field calculator says that at 10 feet, 100mm, f/2.8, I should get 0.33 feet, and at 5 feet, 50mm, f/2.8, I should get 0.33 feet. If depth of field is the same, why do the lights in the background produce larger circles in the 100mm picture?
The answer is that, while the circles may look bigger, that's a product of the extra magnification given by the 100mm lens. If we take the background from the 50mm image, blow it up, and compare it to a similar crop from the 100mm image,
we see that they look nearly identical.
Depending on how you look at it, you can take two conclusions from this:
Conclusion 1: A telephoto lens produces larger circles from points of light in the background because of its larger magnification.
Conclusion 2: The circles made by points of light in the background are the same size, relative to the size of the details in the background. You could say that the background is just as "out of focus" in both -- that is, if you had a sensor with infinite resolution, you wouldn't get any more information about the background out of either image.
Originally by user378. Source · Licensed CC BY-SA 4.0
user378
15y ago
0
Generated from our catalog & community — verify before relying on it.
For the same sensor size, framing, and f-number, depth of field is approximately the same. So your 50mm at 4m and 100mm at 8m example should give very similar DOF on the subject.
But “subject isolation” is broader than DOF. A longer focal length can make the background look larger and more compressed, which often makes blur appear more prominent in the final image, even when the underlying DOF is similar. So in practice, a telephoto setup often gives a stronger isolated look.
A major factor is background distance: the farther the background is behind the subject, the blurrier and less distracting it becomes. In real shooting, the most effective ways to increase isolation are:
- use a wider aperture (lower f-number)
- move closer to the subject
- place the subject farther from the background
- use a longer focal length when it improves composition/background simplification
Other isolation tools include lighting, panning, tilt effects, careful background choice, or selective blur/sharpening in post.
Rule of thumb: if you want your subject to “pop,” prioritize aperture, subject distance, and subject-to-background distance before worrying about small focal-length changes.
Recommended products
UniqueBot
AI15y ago
Your Answer
Related Questions
Does depth of field stay the same if framing and aperture are fixed but focal length changes?
Does a longer 1:1 macro lens give more depth of field at the same magnification?
Does focal length change depth of field if subject size stays the same?
Does background blur change with focal length when subject framing stays the same?
Does an 85mm lens give more background blur than a 50mm if framing stays the same?