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Forum: Pentax SLR Lens Discussion
08-17-2017, 10:15 AM
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No, the 70mm will have a stronger blur. The strength of the blur depends on both focusing distance and focal length, but the focal length has a stronger effect over small changes in distance. It's subtle (and I used to have a really good example but deleted it from my Flickr), but the 70mm will be slightly stronger in blur than the 50mm.
If you use a short focal length, you'll have to get a lot closer but the blur will not be nearly as strong. Here's a non-perfect example, both shots at f/2.8:
At 31mm:
At 77mm:
Mari's position in the frame is about the same for both of these, but obviously I am a lot closer to her than I am with the 31 than the 77. Some differences:
- Everything in front of her is in good focus in the 31mm shot. You can see some slight blurring with the 77 in the forest ground
- There's some definition in the waves in the 31mm shot; you can see lines in the waves in the 31mm shot and in the 77 shot, these are far less defined
- The biggest differences are in DoF and right behind her. The sand behind her is mostly blurred out in the 77mm shot and in the 31, you can see well-defined footprints and tracks in the sand
This isn't a perfect comparison because the angles are different and there was no attempt made to get equal magnification (just happened to have two similar shots), but it does show you how focusing closer isn't quite enough. You need a long focal length and have to focus closely.
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Forum: Pentax SLR Lens Discussion
08-14-2017, 10:33 AM
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We can calculate this! I did a test involving a shelving unit. With the 77mm lens, I had to be 12ft away to capture it. At 55mm, I had to be 9ft away. The K-1 was used.
At f/2.4 with the 55mm: 8.64ft to 9.61ft for 1.15ft of DoF
At f/2.4 with the 77mm: 8.71ft to 9.31ft for 0.59ft of DoF
The thing to remember is that the DoF depends nonlinearly on the focal length and other parameters:
Depth of Field Equations
What will also change with focal length is the strength of the blur. It will be stronger at higher focal lengths, even though you have to move back farther.
Just FYI, with the FA77, I need to be about 12-15 feet away from an average woman (5'2" to 5'5", maybe as tall as 5'9" or 5'10" with heels) to capture her head to toe if she's standing. Mari here is probably about 5'4" or so and I was 4m away (13.2 ft) according to Photoshop when I took this:
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