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What are the benefits of a constant aperture lens?
  • It seems like all of the high end lenses are constant aperture. What are the benefits and drawbacks of a constant aperture lens?

    The biggest benefit I can think of is that the exposure will stay constant as you zoom. You can fix the shutter speed, focal ratio, and ISO values and then zoom without needing to change them. That sounds great but do we really need that?

    With cameras that have Auto ISO available in manual mode can’t you just let the auto ISO control the exposure while you zoom?

    Also aren’t all lenses constant aperture, just not always at their wide open apertures? If you stop down to the minimum aperture of the lens then the aperture will remain fixed in aperture priority or manual mode while you zoom even if the lens isn’t a true “Constant Aperture” lens.

    What happens as you zoom? Aren’t these lenses still changing the aperture in order to keep it constant? Doesn’t that mean that your depth of field will change as you zoom? If the focal ratio of a not constant aperture zoom lens increases as you zoom in could it then keep depth of field constant as you zoom if so desired? Would that ever be desirable?

    Couldn’t a constant aperture zoom lens have a slightly wider aperture or slightly longer focal length if it wasn’t a constant aperture lens wide open? Aren’t they crippling the lens slightly in order to make it constant aperture wide open?

    Also what about being parfocal. Isn’t maintaining focus as you zoom more important than maintaining focal ratio especially in cases where you can let Auto ISO solve the changing exposure issue?

    I am just curious as to why we insist on having constant aperture lenses. To me the benefits of a constant aperture lens do not outweigh the benefits of a non-constant aperture lens as long as your camera allows auto ISO in the modes you are shooting with.

    With that being said the 35-100mm F2.8 is pretty much the only lens I use on my GH4 now. However, I really don’t zoom that often while filming so I am not sure I use it to its full abilities.

  • 4 Replies sorted by
  • Using auto ISO is an option, but with any kind of auto exposure the exposure can change as you move the camera around. The exposure shift is reactive - it happens after something brighter or darker comes into the frame. To me this screams "amateur". Compensating for auto exposure in post is not easy.

    Most zoom lenses are variable aperture. It's not just a question of the maximum aperture (smallest f-number setting). Regardless of the aperture setting, the f-number changes as the lens is zoomed, making the image brighter or darker.

    Some constant aperture zoom lenses are not constant near the maximum aperture setting: the f-number gets larger as you zoom in. But they're still considered constant aperture. Most large aperture TV zoom lenses with long zoom ratios are like this.

    The advantage of a constant aperture lens is, just as you said, so that the exposure does not change as you zoom in or out. If you zoom in or out during a shot, maintaining constant brightness is critical for having the footage look good. If you only zoom between shots, having a constant f-number is merely a convenience so that you don't need to adjust the shutter speed, aperture setting, or ISO setting to compensate and try to maintain a constant exposure.

    As for performing exposure compensation in post: In a standard video gamma color space, you can't do exposure compensation. In linear color spaces like raw and ACES, you can and it's not difficult. But it's much easier to compensate for the changing brightness of your scene than it is to compensate for the exposure shift of a variable aperture zoom lens.

    I have piles of constant aperture zoom lenses. Even when I don't zoom during shots, the convenience of not having to adjust the exposure is tremendous. I also have variable aperture zoom lenses which I only use for photos. Most of my photo shooting uses auto ISO, so the exposure shift is not so much of an issue.

    The depth of the focus field is a function of several variables. It would be very difficult to make a zoom lens with constant depth of field, and I don't think any lens has this. I'm not sure it would be desirable even if it were possible. For all zoom lenses, depth of field decreases as you zoom in.

    Making a zoom lens have a constant aperture is a design trade-off. To maintain a constant f-number, the entrance pupil diameter must increase with the focal length. Achieving that usually makes a lens larger and heavier than it would be if it were variable aperture. That's why photographic zoom lenses are usually not constant aperture. But TV zoom lenses are almost all constant aperture.

  • It's all about maintaining exposure. Some of the variable aperture zooms are stopless, where it arguably matters less. But a lot of them step down exposure as you zoom in, which just looks bad. And if you are using auto ISO, even with a stopless variable aperture zoom, you'll see the image grow darker, then jump back, then get darker again, then jump back. Not good.

    You are right, depth of field will change as you zoom in. This is true of all zoom lenses, constant aperture or not. It happens all very smoothly though, and I've never heard of anyone seeing this as an issue.

    I think you are right though, that constant aperture zooms have had their max aperture reduced on either the wide end or the long end, to allow for constant aperture. I don't know if this could really be called crippling.

    Personally, I think controlled zooms have an important place in the filmmaker's toolkit. A lot of filmmakers in the 60s and 70s did extremely long zooms. I love this aesthetic, but I would only ever attempt it with a constant aperture zoom.

  • Personally I prefer a software zoom over a mechanical zoom if you aren’t zooming very far and you have 4K footage that will be down sampled to 1080p anyway. However, I know that isn’t always the case.

    Have you tried the Auto ISO with the GH4 for Video? I will admit that I don’t really zoom that much in video so I haven’t fully tested it yet. However, I think you will find that it is actually quite good for maintaining exposure smoothly.

    In my experience the micro adjustments that the camera does because the lens isn’t parfocal are much more distracting than the micro exposure changes.

    Another thing is that I don’t ever deal with controlled lighting. I am always shooting in inconsistent lighting run and gun style. Manual exposure is unrealistic in those situations because it is constantly changing. Auto ISO is perfect though. I guess I have just become accustomed to it. However, I think Auto ISO in the GH4 works very well.

  • Is the auto ISO on your camera setting stopless/continuous? If not, it seems like on a f/3.5-5.6 zoom, you'd see 4 distinct steps (assuming it adjusts 1/3 stop at a time) while zooming in as the ISO adjusted to compensate. If it's continuous, that might make a decent compromise.

    As someone previously mentioned, I'd prefer the convenient of not having to re-meter a shot after zooming in.