Posted: 7/22/2025 6:16:52 AM EDT
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Didn't this forum used to be camera something? Oh well. I am putting together equipment for digiscoping. I want to be able to run a DSLR in live mode and record it on a notebook to spot shots at a mile. Light gathering is a significant consideration. 50mm seems to be the recommended focal length and I have one that is f1.4. Since f stop is a calculation of focal length and aperature, and a 55mm f1.4 would have a physically larger aperature, would it let in more light? Or, to make the question a little more obvious, would an 85mm f1.4 pass more light than a 50mm f1.4? Thanks! |
N&MEM, SSDR, NRA Life Member
Epstein Scalia did not kill himself.
Gun control is literally Hitler.
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Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?
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I had tried searching several times and the responses were all 50mm vs 85mm, which is better for this or that. Searched again using different phrasing and got this AI response. It may be ther first time I ever got a completely accurate AI response. Usually they get something whacked out. No, assuming both lenses are set to f/1.4, they would theoretically pass the same amount of light to the camera's sensor for a given scene brightness. Here's why, F-Number and Light Transmission: The f-number (or f-stop) represents the ratio of a lens's focal length to its aperture diameter. A lower f-number indicates a larger aperture opening, allowing more light to pass through the lens. Aperture and Focal Length Relationship: While both lenses are set at the same f-number, their physical aperture openings will differ due to their differing focal lengths. For instance, a 50mm lens at f/1.4 will have a smaller physical aperture diameter than an 85mm lens at f/1.4. This is because to maintain the same f-number as the focal length increases, the diameter of the aperture also needs to increase proportionally. Equal Brightness, Different Magnification: The f-number system was designed so that different lenses set to the same f-number will produce images of the same brightness (exposure) from a given scene, regardless of their focal length. However, the key difference is that the longer focal length lens (85mm in this case) will provide greater magnification, meaning it will capture a narrower field of view compared to the shorter focal length lens (50mm). In essence, while the amount of light collected from the entire scene might differ due to the change in field of view, the brightness of the projected image on the sensor at a given f-stop remains theoretically constant across different focal length lenses. |
N&MEM, SSDR, NRA Life Member
Epstein Scalia did not kill himself.
Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?
Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?
| So while the 85mm f1.4 would let in more light in the absolute, which is what I was thinking when I posted the question, the relative brightness of the image would not change. |
N&MEM, SSDR, NRA Life Member
Epstein Scalia did not kill himself.
Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?
Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?
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Whether it's 85 or 50mm, a 1.4 aperture is going to result int the same shutter speed for any given ISO setting. But, I'm just not following about why you'd need to be shooting wide open like this. Outside of dusk or dawn, I just don't think you'd ever be opening wider than f/2.8 or f/4 at a regular video rates of 1/30th or 1/60th sec. |
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In general, the image becomes clearer and sharper as you stop down, even though there's less light. Most landscape photography is done at f/8-f/11 or lower. Most video work is done around f/5.6-f/8. I'm not sure of the application here. If you are trying to use a camera for rifle scope work, I would think a longer telephoto lens which gets your target area larger in the image would be better than a fast prime used for portrait work. |
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1080P, 60 fps. There will be a considerable amout of glass in front of the camera and up to 60 or 70X zoom. I will be spotting, or at least trying to spot a 6.5mm or 7mm bullet splash at one mile and 2,440 yards, perhaps further, in soft dirt. The rifle scope is 10 - 60X. You can always stop down if you need to but you cannot 'stop up' a f5.6 lens to f1.4. |
N&MEM, SSDR, NRA Life Member
Epstein Scalia did not kill himself.
Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?
Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?
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I have seen multiple telescopes that attach directly to the camera with an adapter. If can't really envision a world where you are attaching a spotting scope in front of the lens. The front element of an 85 f1.4 is pretty sizable. If you are trying to have that lens look into a viewfinder, that is going to be tough. -shooter |
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Originally Posted By shooter220: I have seen multiple telescopes that attach directly to the camera with an adapter. If can't really envision a world where you are attaching a spotting scope in front of the lens. The front element of an 85 f1.4 is pretty sizable. If you are trying to have that lens look into a viewfinder, that is going to be tough. -shooter Here is an illustration from Kowa showing the adaptors they make to connect a DSLR to their spotting scopes. 
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N&MEM, SSDR, NRA Life Member
Epstein Scalia did not kill himself.
Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?
Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?
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I did a tiny bit of playing on their website - I don't think they have an adapter that works for the Nikon 85 f1.4. It looks like they go as high as 72mm, and that lens requires a 77mm filter. If you are buying a lens for the application, I would double check filter size specs, and mock-up the sequence of things you need to make sure all the parts are available. The arrangement in their image kind of horrifies me - I hope they manage the tolerances very carefully. The adapters certainly aren't cheap, so that gives me hope. -shooter |
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Originally Posted By SteveOak: The filter size of the Nikon 85mm f/1.4 AI-s is 72. Interesting - I had no idea. But I also haven't had my hands on one of those in decades. If you are buying, looks like there are some good deals to be had on them as well. The image quality on the AI-S should be very good. |
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Canon makes a 50mm f1.2 (EF mount). There is also the insane 50mm f1.0, but rare and hard to find. With modern equipment, unless you're very worried about getting bokeh from a super wide aperture, your camera will function pretty well in low light.. a far cry from the days of pushing Tri-X to 1600asa. Edit. Are you not permitted to use a long telephoto lens? Because if you can, you can rent one if you're not in the market to buy (I say that because I would love one of Canon's 1400mm lenses, but that's like buying a car!). |
Gnu User, Clumsy Migratory Off-topic Herd Ungulate
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Just so I'm clear, you intend to place a DSLR behind a spotting scope? If that's the case, don't throw money at a f/1.4 lens. Or, if you do, at least rent one (and an alternate like a 35mm f/1.8 that will save you loads of cash and probably achieve the same results) and see how it looks. |
ProTip: Never pick a fight with a tree planter. He swings a hoedad for 9 hours a day, carries 30-40 pounds of seedlings on his back, and has 12-14 friends just like him hiding out on the other side of the junk butt pile. They will fuck your shit up.
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Originally Posted By FredMan: Just so I'm clear, you intend to place a DSLR behind a spotting scope? If that's the case, don't throw money at a f/1.4 lens. Or, if you do, at least rent one (and an alternate like a 35mm f/1.8 that will save you loads of cash and probably achieve the same results) and see how it looks. Yes. Please explain. A 35mm lens would have an effective lens length of 35 X 60 = 2,100 mm. I already have a 50mm f1.8 would has an effective lens length of 50 X 60 = 3,000 mm so a 35mm would be a significant step down. |
N&MEM, SSDR, NRA Life Member
Epstein Scalia did not kill himself.
Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?
Gun control is literally Hitler.
Trump's actions? Watergate does not bother me, does your conscience bother you?