Lower End Macro Lens for 1000D/500D?

Started Nov 3, 2010 | Discussions thread
Steve Balcombe Forum Pro • Posts: 13,090
Re: Lower End Macro Lens for 1000D/500D?

ecka84 wrote:

Gautam Majumdar wrote:

For macro photography at same magnification DoF depends only on the aperture. Focal length has no bearing on it.

Maybe so, but apertures like f/2 or larger are not really usable for macro photography. I was talking about using the lens for portraits, obviously. 10-14 feet is far from macro distance :), while the longer focal length makes a nicer background isolation (bokeh).

Your calculation is still wrong. If the 60 mm lens is used at 10 feet, the 100 mm lens will need to be used at 100/60 = 16.67 feet for the same framing. At f/2.8 the calculated depth of field (using dofmaster) is 0.89 ft in either case. So the 60/2 has one stop more depth of field control.

However distant background blur depends only on aperture size so the difference in maximum blur is 60/2 = 30 mm vs 100/2.8 = 35.7 mm. You can express this difference in stops if you wish (it's about 1/2 stop) but what it boils down to visually is the maximum blur circle of the 100/2.8 lens is 19% more than the 60/2.

Interestingly the 100 mm lens's advantage diminishes if the background is closer. You have to do a proper blur circle diameter calculation to measure this, but if the background is at 50ft for example (with the above subject distances), the 100/2.8's blur circle wide open is only 9.9% larger than the 60/2.

If you measure the background blur at the same f-stop, as you would if you need the same depth of field, then the difference is proportional to the focal length so the 100 mm lens wins by a substantial 67%.

The calculations ignore any focal length reduction due to internal focusing and also the effect of lens asymmetry (pupil magnification) but I think we can assume that these errors will be small at these distances and roughly similar for both lenses.

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