Stupid question about max aperatures

Discussion in 'Beyond the Basics' started by sothoth, Feb 9, 2007.

  1. sothoth

    sothoth TPF Noob!

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    I have a Sigma 10-20 f/4.5-5.6. I also have a Sigma 28-70 f/2.8. The 28-70 is much better for indoor shotting due to the larger max aperature: when I use the built-in flash indoors, the 28-70 looks great, but the 10-20 are very underexposed.

    My question is why. Since this is an SLR (metered through the lens) and the photo subject is just a few feet away, shouldn't the camera compensate for the smaller max aperature on this lens? Or is this a problem with the built-in not being able to produce enough light to compensate for a smaller aperature? It seems to me like the SLR should "know" what aperature I'm at and adjust for the limitations of the flash by changing ISO or using a longer exposure. Maybe it's just me but I find this perplexing and kind of annoying.

    Next questions is will buying a mounted or non-mounted external flash solve this problem? I know the built-in is rather sucky, but I'm not convinced that a brighter/better flash will take care of this. Or is the best solution for me to adjust the settings on my own and forget about wanting the camera to take decent indoor shots without my intervention.
     
  2. Don Simon

    Don Simon TPF Noob!

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    Assuming you are not using manual exposure settings, the camera's light meter is likely to be getting different readings due to the wider metered area and therefore exposing differently. You could try changing the metering mode to center-weighted or spot, using exposure compensation, or simply using a handheld meter and setting exposure manually.

    Also I'm not sure a built-in flash is designed for use with a lens as wide as 10mm.

    Finally, I would not rely on the camera to take decent shots of any kind without your intervention. Cameras now have complicated metering systems which are very clever and at the same time very dumb. At the end of the day it's still you who has to take the shot.
     
  3. JIP

    JIP No longer a newbie, moving up!

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    Are you using manual exposure? I do not want to assume anything about your abilities but if you are using manual you still need to meter somehow. there is a meter in your camera and it will tell you the proper exposure so If you are using manual and just setting your camera on some arbitrary setting that you want and ignoring the cameras meter you probably will get an underexposure.
     
  4. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    The built in flash covers a much smaller angle of view than what you are getting with a 10-20mm lens. If you are using a DSLR it's probably designed to barely cover the 20mm angle of view. If you are using a 35mm SLR, it's probably designed to go as wide as a 28mm lens, but no wider. You need something to spread it out, like a diffuser panel.

    An external flash will have the same issues, although it might come with a wide angle diffuser. Or you can buy something like a Sto Fen Omnibounce, GF Lightsphere, or make your own out of tissue paper or transluscent plastic.
     
  5. fmw

    fmw No longer a newbie, moving up!

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    ..........or, best of all, turn the on camera flash off never to be used again. Get an external flash unit and an extension sync cord and do flash photography the way it should be done. You really can't make good images with that dead on, deer-in-the-headlights, subject-surrounded-by dark-shadow, red-eyed look. Even pros can't do it with on camera flash.
     
  6. sothoth

    sothoth TPF Noob!

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    I thought you might chime in and say that... :)

    And it's good advice, so I'm not making light of the message (no pun intended).

    I guess the explanation is the wide angle causing the problem since the flash focuses the light to a smaller area. I'll try a few things to compensate, including the center metering and manually overexposing. Neither of those are a problem to do, I just didn't think about the light being too narrowly focused for such a wide angle (its on a dSLR with a 1.6x correction factor so it's really a 16-24). I suppose I'll still have the center of the image more exposed than the edges but it sounds like that's unavoidable. Maybe I'll stick with my 28-70 f/2.8 and forget about it.

    FMW, if you have an extra one of those flashes you can give me, I'll send you a private message with my shipping address. :)

    Thanks for the feedback from everyone.
     
  7. JIP

    JIP No longer a newbie, moving up!

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    I just googled the 10-20 and all I can find is a digital lens so if thats the one you have it is already corrected so on your DSLR it is a true 10mm no crop factor
     
  8. sothoth

    sothoth TPF Noob!

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    My mistake...
     
  9. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Corrected for what? What is a "true" focal length?
     
  10. Don Simon

    Don Simon TPF Noob!

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    I guess 'corrected' could refer to the extra coating applied to lenses designed for digital sensors. But the main point about "digital" lenses is simply that they are designed to cover a smaller area. I have yet to see a benefit to this except perhaps the lenses are cheaper to make. In fact there may be a benefit to using lenses designed for 35mm with a smaller sensor, because you're not using the outer edges of the lens. Whatever the case, a 10-20mm is not really a 16-24mm (wouldn't it be a 16-32 anyway?), it is a 10-20mm. A true 10-20mm, really a 10-20mm. The 'crop factor' is simply a way of comparing the effective field of view of a given focal length with film/sensor of different size.
     
  11. fmw

    fmw No longer a newbie, moving up!

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    The issue relates to wide angle lenses. There is a benefit to using longer lenses designed for 35mm cameras on digitals. The reason is that it crops out the corners and edges where most of the design compromises are visible. In effect, you use just the sweet spot of the lens. With wide angles, however, the "crop factor" affects the angle of view in a very negative way, turning wide angles into something not so wide.

    Making a 10-20mm zoom for a 35mm camera would be nearly impossible. The corners of the frame would be stretched beyond all recognition. 20mm itself is quite wide on a 35mm frame. 10 would be a 180 degree fisheye. You can see the problem. But designing the 10-20 zoom for a digital frame is like designing a 15-35mm zoom for 35mm. No problem - well certainly possible at any rate.

    Wide angle photography was very popular in the 35mm days and less so in these digital days because there are so few wide angle options for digital cameras. I encourage the manufacturers to catch up or, as an alternative, make all the sensors in 35mm format so we can use available wide angles.
     
  12. Don Simon

    Don Simon TPF Noob!

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    I've noticed the benefits of using lenses designed for 35mm on small-sensor dSLRs; often noticeably less corner softness. I seem to remember someone recently suggesting there was an inherent benefit to "digital-only" lenses but I can't imagine what it would be; one argument I've heard is that digital sensors are more sensitive to reflections and therefore the lenses needs different coating, but I've never noticed any aberrations using older glass with dSLRs that I wasn't already experiencing with film.

    I agree completely on the point about wide-angles. I still shoot 35mm film for several reasons, partly because I enjoy it, but one factor is that I can do wide-angle photography with 35mm film while the cost of lenses providing an equivalent field of view on the dSLR is completely prohibitive. I wish there was more choice (and more affordable choice) of wide-angle glass, plus more standard primes as opposed to telephoto lenses of which there seems to be no end of choice. I consider it fairly irritating that most companies have so comprehensively dumped 35mm film, and yet left a big gap in their systems in this way.
     

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