'Apparent' Focal Length

Discussion in 'Medium Format' started by m_allegretta, Aug 27, 2021.

  1. How does one calculate the 'apparent' focal length of a 1000mm Hasselblad optic when adapted to a 35mm Full Frame Camera?
  2. Steve is actually right.
    In an important sense, a lens is simply whatever focal length it actually is.

    Some times the term equivalent is used when comparing lenses from one image-size format to another.
    In that sense an 80mm lens made for a 6cm image, will be a rough equivalent of a 50mm lens for a 24x36mm format camera, that is a NORMAL focal length.
  3. m42dave

    m42dave Dave E.

  4. While its true that focal length is focal length, i.e. 1000mm is 1000mm whatever the frame format, the human mind tends to balk at this and attempt to interpret the coverage based on whatever format a photographer considers their primary orientation. In this case, if coming at the question from the baseline of a Hasselblad-oriented photographer switching to 35mm film / FX digital, the 1000mm "Hasselblad lens" on the smaller camera would have the apparent angle of view of a 1600mm lens on the 'blad (more or less, theres a tricky fudge factor involved due to the difference between square and 2:3 rectangle framing).

    Turned around the other way, the coverage of a 1000mm lens mounted on the Hasselblad would appear equivalent to approx 600mm thru the eyes of a 35mm-SLR-oriented photographer. Again, theres a variable fudge factor depending how one uses the 6x6 frame (full square, cropped to 645, or cropped to 2:3 to match the smaller format ratio).
    Last edited: Aug 28, 2021
  5. Interesting question : how would you !measure! the focal length of one of your photographic lenses ? Let's say a retrofocus wide-angle lens to make it a little difficult.
  6. Set to infinity focal lengths are indeed what they claim to be.
    Some lenses with internal focusing lose FL, when focused close. 70-200s behaving like a 135mm when taking tight headshots at their long end come to mind. If the question is about THAT issue:
    Take stated FL as available bellows draw and apparent FL as whatever provides your magnification ratio at that draw. Or stack 50cm of extension tubes to take a 1:2 macro shot at true 1000mm.
    FTR: I don't know your Hasselblad lens' construction.
  7. Thank you for your valued perspectives. The D700 sensing the center portion of the 6x6 image would give the 'impression' of a longer FL than 1000mm pictured lens setup. The estimated "apparent" 1600mm FOV generated by the D700 is reasonable. I don't have a 1600mm f/16 telephoto lens for 35mm film camera. Leveraging the good quality Hasselblad V optics I have on hand is a viable option for the few times I may need that reach by using the FX DSLR. Admittedly, I can crop the 6x6 scanned film image to get the same, but the D700 image is available faster and cheaper.
  8. It perhaps helps to see lenses as lenses, with their own lens properties. Focal length is one of those.
    And there are cameras and sensors, with their own properties. Focal length is not one of their properties.

    So a, say, 1000 mm lens always is a 1000 mm lens.
    When compared to other 1000 mm lenses, it is the same focal length.
    Compared to other lenses that are shorter or longer in focal length, it is longer or shorter in focal length.

    Do you have a 1000 mm lens for camera X? A 500 mmm lens combined with a 2x converter, both made for another camera, Y, will behave as that brand X 1000 mm lens. No difference (in focal length, angle of view, coverage, focussing distances).
    Ed_Ingold likes this.
  9. I'm not sure how useful that 1000mm f/16 combo is on any camera, but the equivalent field-of-view between formats is easy enough to work out. It's simply format1/format2 * focal length.

    Your 'blad frame has a width (and height) of ~56mm. Divide that by the 36mm across a Nikon frame, and you get 1.555'. So you'd need a 1,555mm lens on the 'blad to get the same width-of-view as on the Nikon.

    Conversely, you'd only need a 643mm lens on a Nikon to give the same width-of-view as 1000mm on a 'blad.

    But it's still a slow 1000mm lens - whatever you stick on the back of it.
  10. Yes, thats the "crop factor" effect vs the same lens' coverage on your Hasselblad 6x6.. The 1000mm focal length stays the same, but the two different cameras get different angles of view from the same lens via their different frame sizes. As it happens, I also use my Hasselblad V lenses on my Nikon D700 occasionally. Pictorially, I find the D700 sensor characteristics an excellent match with the V lens characteristics.

    Of course the handling can be clumsy, and with shorter V focal lengths one sometimes needs to consider the true nature of each optic (i.e., the 50mm Distagon V mounted on the Nikon may be an apparent "normal 50mm" due to the crop factor, but remains a retrofocus wide angle in design with the usual optical properties and compromises of such designs). These optical category variations between the V lenses and identical focal lengths native to the smaller format become insignificant with lenses longer than the 80mm Planar, where the only change when mounting the V lens on a Nikon/Sony/Canon etc would be the crop factor.

    Be sure you are interpreting the crop factor and "lens equivalence" correctly, Your Hasselblad 500mm APO + 2x Mutar don't become "a 1600mm lens" when mounted on your Nikon: rather, the combined APO + Mutar on Nikon give you the equivalent angle of view/ frame coverage of a 1600mm lens on your Hasselblad. Put another way, 1000mm on Nikon gives you a magnification reach of 20x the standard Nikon 50mm focal length, which translated to the 6x6 Hasselblad is 20x the standard 80mm focal length (1600mm equivalent). The focal length is fixed at 1000mm, only the angle of view changes, but its convenient mental shorthand to think of these changes as being different focal lengths on the different cameras.

    It would depend somewhat on your subject and the shooting environment. The 500mm Zeiss V APO is an excellent lens, but at f/8 is a stop or more slower and far larger/heavier than 500mm lenses designed for the smaller format. The Zeiss Mutar is a good teleconverter, but using it to make the 500mm into 1000mm pushes the envelope in terms of its effect on lens performance, plus drops the effective max aperture down to a truly dismal f/16. Such dim viewing/focusing combined with the size/weight could lead to practical drawbacks that might make other alternatives preferable for subjects other than completely static architecture in bright daylight etc.

    Beyond a certain point in digital tele photography (typically 400mm on a Nikon D700), getting good quality pixel density with the longest reach in the most compact AF package tends to more predictable, usable results. The DX sensor format often wins out here, where mounting a decent but affordable 500mm DX/FX optimized AF lens on an affordable DX camera with good 24 MP sensor (say Nikon D7100 and later) nets you the effective reach of a 750mm lens on the D700 (with double the pixel density). Moderate cropping would get you the reach equivalent of 1000mm on the D700, again in a much more practical maneuverable package. Something to consider, if you find yourself increasingly drawn to long tele work with varied subjects on Nikon.
    Last edited: Aug 29, 2021
  11. I find "reach" a terrible concept, that should be banished.

    You apparently have a Hasselblad (plus 500 mm lens). Take a picture on 6x6 format using that or any other lens. Change film magazine to 6x4.5 format. You don't think in doing so, you also change the focal length (real, effective, equivalent or whatever), do you?
    Take either 6x6 or 6x4.5 format image and crop it to 16x24 mm. Why would doing that make anyone question the focal length of the lens that was used? It is still the same. There is no point in starting to think about effective or equivalent or whatever other variant of the parameter focal length, just because you crop to a tiny portion of the format you were using before.

    "Reach"... What will i have gained cropping a larger format to a (much) smaller format? What is that thing "reach" but a misnomer for the smaller crop, the lesser image?
    Yet people treat it as if it is some kind of advantage. It's no more than the confusion called "crop factor" squared.
  12. Thanks for discussing the FL interpretation further. I agree the FL is 1000mm and is unchanging when switching formats, In this scenario, the D700 body is simply cropping. Below are two images made with the setup Hasselblad 500/8 CF + 2X Mutar on D700; both made at ISO 400. These JPEGs are generated from unprocessed NEFs.

    Bird was positioned at about 6m. Moon focus was checked with "Live View". I think the images are good and reasonably sharp.


    orsetto likes this.
  13. "Is the glass half-full or half-empty?" The term "effective focal length" depends on your frame of reference. Since about 1960, that frame of reference has been full frame 35 mm, 36mm x 24 mm (actually double frame per cinematic usage). Although there were a number of sub-FF film formats in the interim, it was not until APS-C sized digital cameras which looked like, took the same lenses, and were used in the same manner as SLRs from the previous four decades, that the concept of "equivalent focal length" gained stature. Again this is relative to a FF 35 mm image, with a 3:2 aspect ratio.

    On this basis, it is safe to say that a 1000 mm lens, regardless of the size of its image circle, behaves the same as any other 1000 mm lens on a FF camera. Most of us who were adults before 9/11 tend to think in these terms when deciding which lens to use in a given situation, or when buying a new lens. Commercial photographers who tend to use larger formats might think differently, or maybe not. When you are task oriented, you tend to "think" in the format you are using at the time. It's much like learning a foreign language well enough to stop translating in your head when you speak or read.

    I used medium format (6x6) a lot In the middle of the digital age, although mainly after I acquired a digital back making it economical and effective. The question became what is the "equivalent focal length" at 4x4 cm v 6x6 (easy, it's 1.5x). I actually never gave much though to the equivalent 35 mm focal length. With medium format, thought is more in terms of wide, normal and long. Since medium format has different aspect ratios than 35 mm film, there are several equally valid (and irrelevant) ways to calculate the equivalent focal length (or cropping factor) - diagonal (traditional), long side, sort side, square root of the areas. Pick one, and the next person will argue for another, ad infinitum.
    Gary Naka and ajkocu like this.
  14. I'm someone too who thinks that the concept of "equivalent" focal lengths has value.

    Like many people, I learned on 35mm film, and now the unfortunate reality is that I shoot more 24x36mm digital than anything else now since I don't currently have a darkroom or otherwise good film handling facilities.

    I can "see" what a 35mm or 50mm or 200mm lens looks like on 35mm film/full frame digital. For that reason, when switching to other formats, thinking in terms of "equivalent focal length" helps me make intelligent lens choices. Yes, I know that my 50mm Distagon isn't EXACTLY the same as my 35mm f/1.4 Nikkor, but I can use the two lenses in similar situations...aside of course from the Distagon being 3 stops slower.

    And when I read this, my thought is "Of course a 1000mm lens on a 35mm camera looks like a 1000mm lens"
  15. While I grew up on 35mm film, I shoot in different formats, both film and digital.
    My current digital are m4/3 and APS-C. So I don't like the "equivalent focal length," referencing FF, since I no longer use 35mm film much.
    For me, if you do "equivalent focal length," it should be a relevant focal length. A format that you use and know.
    There is more than a generation of photographers who have no idea what you are talking about, when you say "FF equivalent." They have never use a 35mm film or FF camera, so they have no reference point. It is like saying the speed limit is 40 knots. :confused:

    Because I shoot in multiple format, for me it is MUCH easier to think in magnification, based off the normal lens for that format. As @Ed_Ingold said, I think in whatever format I am using, and my reference is the normal lens for that format.

    Focal length of the lens / normal lens = magnification.
    • on 4x5, 1000mm / 150mm = 6.7x
    • on 6x6, 1000mm / 80mm = 12.5x
    • on FF, 1000mm / 50mm = 20x
    • on APS-C, 1000mm / 35mm = 29x
    • on m4/3, 1000mm / 25mm = 40x
    As you can see 1000mm is 1000mm, regardless of format. What is different is the magnification, based on the normal lens for that format.
    Last edited: Sep 5, 2021
    m_allegretta likes this.
  16. I'm not sure how useful the concept of magnification is, especially relative to a "normal" lens. Magnification is very difficult to visualise for one thing, and for another the 'normal' lens focal-length is based on no particular standard. (It's loosely based on the format diagonal, which is biased by the aspect-ratio.) For example; the true diagonal of the 35mm format is 43mm, and that of 6x6cm is 79.2mm.

    Much better to take the horizontal or vertical field-of-view as a comparator. So for a 1000 mm lens @ 100 metres the approximate horizontal fields are:
    • 4"x5" - 12 metres
    • 6x6cm - 5.6 metres
    • 36x24mm - 3.6 metres
    • APS-C - 2.4 metres
    • Micro 4/3 - 1.73 metres
    These fields are fairly easily calculated by dividing the subject distance by the lens focal length and then multiplying by the required width/height of the format frame size.

    The above table aligns reasonably well with the magnification table, but actual measurements are much more easily visualised than a magnification factor.
  17. Agree on the horizontal field of view. If I am trying to fit an image into a frame, it is usually the horizontal axis (and sometimes the vertical) that I am concerned with.
    For me, the diagonal of the image frame is not a practical measurement. I have yet to have a need to do a diagonal.

    For me, in practice, just looking at a scene, magnification difference is hard to tell beyond about 8x. My eye can't visualize a slice that small and see what is in that slice.
    If I'm looking through a normal lens,
    • I simply divide the image in the viewfinder into quarters, and take one of the quarters as a 2x magnification over whatever lens I am using. That is easy.
    • The next step is a quarter of a quarter screen for 4x
    • The next step is harder, a quarter of a quarter of a quarter, 8x. That is a small portion of the screen.
    • Beyond 8x, is too hard for my eyes.
  18. Ask any three people how to measure the "equivalency" of lenses between formats and you will get five answers.
    Gary Naka likes this.
  19. I've been using horizontal angle of view for a while when comparing between formats.

    In virtually every case, it's the single most important value for me in choosing a focal length. I consider diagonal nearly useless other than as a very rough comparison. Horizontal allows me to say "I'd use this like I would a 28mm lens on 35mm" or whatever(aside from the fact that I almost never use 28mm lenses...).

    I am fine with crop factors when the aspect ratio is the same, such as comparing APS-C and full frame 35mm. They're simple and straight forward, and it's easy for me to say that my 200mm lens is going to give the field of view of a 300mm lens. When you move to formats with more logical aspect ratios than the wide and short of 35mm film, that kind of falls apart(although I do appreciate when cameras can do something like mask a 4x5 aspect ratio area in the viewfinder..).
    rodeo_joe|1 and m_allegretta like this.

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