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Crop factor vs lens hood size

Discussion in 'Adapted Lenses' started by jai, Jul 8, 2013.

  1. jai

    jai TalkEmount Top Veteran

    589
    Feb 4, 2013
    I recently dropped my camera, and smashed the lens hood off one of my Rokkors. I guess I should be grateful it was on there, and the lens didn’t cop it.

    I am looking to replace it, but it struck me as being too small anyway. With the crop factor sensor, should I be getting a hood designed for a longer focal length lens?

    For example a hood designed for a 35mm, to use on a 24mm? Or is that dumb?
     
  2. davect01

    davect01 Super Moderator

    Aug 20, 2011
    Fountain Hills, AZ
    Dave
    Huh?

    Are you asking if you can fit a larger lens hood? The diameter of the lens where you screw into does not change, but you can get wider, deeper hoods.
     
  3. Deadbear77

    Deadbear77 TalkEmount Hall of Famer

    Sep 14, 2012
    Northeast Ohio
    Kevin
    I never thought of that.


    Sent from my iPhone using TalkNEX mobile app
     
  4. Jefenator

    Jefenator TalkEmount Top Veteran

    876
    Nov 23, 2012
    Oregon, USA
    Jeff
    Well, 24mm on these crop sensors is "35mm equivalent", so I think that actually makes pretty good sense.

    OTOH, I wonder how much good those old-school round hoods do anyway (unless the stray light source is close to a corner).

    I've always disliked working with hoods - FWIW I usually get good results using my left hand to block out the sun when it's just outside the frame. But that's just me...
     
  5. Deadbear77

    Deadbear77 TalkEmount Hall of Famer

    Sep 14, 2012
    Northeast Ohio
    Kevin
    Hmmm for outdoor use with bright sunlight I would rather have a black cape over my head with a hole cut out for the lens.

    That's the only thing I don't like about the evf.


    Sent from my iPhone using TalkNEX mobile app
     
  6. jai

    jai TalkEmount Top Veteran

    589
    Feb 4, 2013
    What I mean is a longer hood. Minolta Rokkors all have 55mm (or 49mm) filter thread sizes, which means you can use them interchangeably. The hood designed for a 35mm f2.8 lens is longer, and not flared as wide, as the one designed for a 24mm lens.

    The one I had for the 24mm lens certainly didn’t look like it did much at all. But it did save my lens from getting smashed!

    I would assume the best hood is the longest hood you can get, before it causes vignetting. The question becomes, does the crop factor mean vignetting is less of an issue than with a full frame (film camera)? That would make sense.

    And if that is true, is it proportional the crop? So a hood from a 35mm lens works perfect on a 24mm prime on a nex? The maths could be more complicated though.
     
  7. Bimjo

    Bimjo Super Moderator

    Oct 28, 2011
    Washington State
    Jim
    Not sure there's a direct formula to determine hood length for FF lenses used on APS-C cameras, but a hood for a FF 35mm lens should work without issue on a FF 24mm lens when used on the smaller sensor.

    Part of the issue is that aftermarket lens hoods are rather generic. They tend to be marked "wide angle", "normal", and "telephoto" which can be less than informative. On the plus side, a longer lens hood will almost always work better than the original hood, and if you manage to get one that's too long you can always trim it if needed.

    If you are more inclined to the mechanical than the mathematical you can proceed a couple of ways. The below assumes round hoods, not rectangular or petal hoods.

    1) If you have access to a bunch of old filters (just the rings actually) you can stack them until the lens starts to vignette and then back off until it stops. That's how long a hood you can use on that lens with your camera.

    2) Take a sheet of paper and form a cylinder from it. Then you put the cylinder over the lens and trim the length until it stops causing vignette in your pictures. When you reach that point measure from the filter threads on the lens to the front of the paper cylinder and that's the longest hood you can put on that lens for your camera. Rough rule of thumb that I use is twice the focal length of the lens and adjust from there.

    3) You can get on ebay and look at all the hoods available. Pick one that looks longer than the one you're using. Buy it, put it on and shoot with it. If it doesn't vignette then use it and be happy. It works at least as well as what you were usiing before.

    It's only rocket science if you want it to be. ;)
     
  8. Bugleone

    Bugleone TalkEmount Veteran

    210
    Aug 21, 2011
    Nearly all lenshoods for old lenses were essentially useless. To 'hood' proeprly you need better purpose designed hoods. The easiest way to make them is to use inexpensive lens caps like this;

    CIMG0983.
     
  9. jai

    jai TalkEmount Top Veteran

    589
    Feb 4, 2013
    Thats the plan. I am not looking at aftermarket hoods, just old minolta hoods that can be purchased solo off ebay.

    So I will pick up one off ebay and report back if people are interested. I think it’s interesting anyway.
     
  10. Poki

    Poki TalkEmount Hall of Famer

    Aug 30, 2011
    Austria
    I always thought the hoods like the one they sell for the RX1 look the coolest, but I guess these 'flower' shaped hoods are more effective.

    More on topic: The hood that comes with the 32mm Touit is afaik smaller (shorter) than the one that comes with the 24mm Sonnar, so I don't know if it's really necessary to get a longer hood just because of a 50% focal length increase.
     
  11. Luckypenguin

    Luckypenguin TalkEmount Regular

    110
    Sep 26, 2012
    Brisbane, Australia
    Nic
    When I was using larger format lenses on smaller format cameras (APS-C, m4/3) I would use cheap step-down filter rings in addition to the existing hood. Two small side-benefits of this was that I could use a smaller lenscap that didn't take up as much room in a pocket and I could use smaller filters (58mm instead of 77mm, for example).