Telescopes vs Telephoto Lenses

As I mentioned here several times, I was always thinking how camera lens’s speed is determined. I was feeling like telescope’s actual speed is a couple of stops faster than telephoto lens’s at camera lens measurement. (or camera is a couple of stop slower at telescope standard) Anyway this must be what all of astro photographers were feeling like. Now it is the time to verify. Fortunately 36ED allows me to compare with my Canon 70-200mm F4 lens(constant F4 speed). Meanwhile 36ED is 200mm/F5.6. So F4 vs F5.6 in same image field. I have set aperture-priority and fixed ISO. So I hope this should be fair comparison.  Since Canon lens’s iris can’t  be controlled from Olympus E-M5 and I didn’t preset Canon lens’s aperture, it was fully open.

36ed_canonE-M5’s metering were indicating same shutter speed!! I was trying to point to several different targets. But both metering were very consistent. This result have made clear my long-time question and proved my speculation was right. As shown at above picture, 36ED is smaller objective lens. But same speed. I guess biggest factor should be the number of lens elements. Glasses attenuate the illumination and their each surface reflects light. As a result, more reduced light power of conventional telephoto lenses is reaching to the imaging sensor. Please note reflection degrades image contrast too. I think this is why telescope’s image looks cleaner and more vivid.

BORG 71FL is 400mm/F5.6 too. It should perform like 400mm/F4 in speed. Moreover, telescopes are quite sharp at the center even at full aperture. If some camera lenses have to be stopped down to achieve same level of sharpness as telescopes’, 400mm/F4 lenses will be further slower than 71FL/F5.6.

71FL astrograph – 280mm/F3.9 should be feeling like fully-opened 300mm/F2.8 telephoto lens in speed, and quite sharp entire full frame sensors at F2.8-like speed.

I’m glad this test cleared the mist in my mind.






~ by tedishikawa on July 30, 2013.

4 Responses to “Telescopes vs Telephoto Lenses”

  1. Hey Ted,

    For astroimaging, which is an even more severe test for optics of any sort, a better comparison with the 36ED optic would be the Canon EF 200mm f/2.8L II stopped down to f/5.6.

    This prime lens becomes a “very good” tool for astroimaging at f/4, according to Jerry Lodriguss, one of today’s foremost astrophotographers. Check his website page discussing camera lenses for AP at:

    The Canon EF 70-200mm f/4 is not mentioned by him for AP use. It also does not have the “L” glass of later model zooms in the same FL range. According to Jerry, the “L” glass zooms “…can perform fairly well, but are very expensive.”

    Since the zoom lens glass is also more complex than the prime’s, the zoom is not the best choice for comparison here.

    For general terrestrial use, your comparison may be valid, but it is not for folks, like myself, who do mostly AP.

    However, I have an older Borg 100ED, f/6.4, fitted with a Feather Touch focuser. It is an EXCELLENT, and relatively lightweight, optic for AP!!

    Best regards,


    • You don’t get my point. I’m talking about “speed”, not total performance.


  2. Hi Ted,

    Folks at dxomark actually measured transmission of many zoom lenses and primes. And it turned out that the number of elements (which is greater in zooms) matters. You can see result for yourself but Canon and Nikon 300/2.8 primes transmit light as if they were F3.2 systems, zoom like 70-200/4 is more like F4.6. And big 200-400/4 Nikon zoom is even darker at F4.9

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