objectivly speaking...........

Discussion in 'Canon Lens Discussion' started by Caladina, Apr 17, 2020.

  1. Caladina

    Caladina Active Member

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    coming from an astronomical telescope background have a big objective lens was one of the things i looked for to gather more light.

    the way camera lens seem (to me) to be measured in light speed is the ƒstop.

    thats ok if you like a thin field of view and chase bokeh, i'm finding i prefer a decent field of view to my images so i probably should be aiming for 2.8 and over in a lens, which brings me to my question.

    does having a huge objective glass in a camera lens make a difference to the field of view vs ƒ stop.
    ie: if i had a lens with an objective of 100mm and i stopped it down to ƒ8.0 be the same as having a 30mm stopped down to ƒ8.0 or would that extra glass width gather more light or more data?

    Do the lenses with the large objectives have them there to benefit them being wide open then the huge glass benefit is lost when stopped down?

    as you guys been around camera lenses alot longer than i have are there any large objective lenses that are standard or standard / zoom you can mention,
    somewhere in the 18 - 130 mm focal lengths be it zooms or primes

    also another thing, i can easily add tubes and converters to my sigma 100-400 but i cant with my sigma 18-35
    would this be the fact the 100-400 was a full frame compatible lens where the 18-35 is a crop only so the rear lens is right at the back of it.
    therefore would it be best to stick to getting full frame lenses for my crop if i want to add tubes to them.
    or do any full frame lenses have less space at the rear because the way the lenses are focused internally?
     

  2. Craig Sherriff

    Craig Sherriff Active Member Site Supporter

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    Caladia, a interesting perspective on lenses, my 3 basic lenses I favor at the moment are 28mm M42 mount, 28-104, EOS mount and a 600mm zoom , FD mount. all are old film lenses which work well on my 1D and my other cameras.
    The conditions for astronomical photography can be summed up in one word, crap.
    Anything worth taking was hidden by mist or clouds, we then had bush fires all around Australia so the haze from that now a lock down due to the virus, light pollution from surrounding suburbs and it's only April.
    Luck should change soon lets hope June will hurry up and the lock down will be finished. I have a 500mm mirror lens I want to try out a Tamron BBar.
    You only have one F stop and that is F8 due to it being a mirror lens.
    Again another old film lens.
     
    Last edited: Apr 17, 2020
  3. Caladina

    Caladina Active Member

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    thanks, the original wasn't ment for taking astro pictures, it was a general inquiry about the workings of the big objectives
     
  4. Craig Sherriff

    Craig Sherriff Active Member Site Supporter

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    May I suggest you direct your inquiry to Johnsey as I believe he would be more technically qualified to answer and he knows more about the lenses than most on the forum.
     
  5. GDN

    GDN Well-Known Member

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    There are a couple of questions that you have here, so if I miss something or I have miss-understood your question let me know.

    But with telescopes, they are no more than a lens without an aperture. A refactor may be measured in inches, so a 6, 8 10, etc, but they will also have a focal length and therefore a f value.

    So to look camera lenses, it is the same. Lets take a 40mm lens as an example. If you follow these links to lenstip

    https://www.lenstip.com/343.1-Lens_review-Canon_EF_40_mm_f_2.8_STM.html

    https://www.lenstip.com/548.1-Lens_review-Sigma_A_40_mm_f_1.4_DG_HSM.html

    If you used both of these lenses next to each other, the image will cover the same field of view. Stop them both down to f8, focus at the same distance, and same things will be in focus. But the lenses are two very different things. The Canon lens you can attach to your camera and walk around all day long without any problems. You could also put your camera and this lens in your large pocket and not worry about a separate carry bag (Canon, how long until the RF mount of this lens?). If you want to attach a filter, it is cheaper to buy, and smaller to carry around. The Sigma on the other hand is none of these. But if you are a shooter in low light, if you like thin depth of field images, this is the better lens. It all depends on your shooting style to what is right for you. This is what takes time in photography as you find your interests. Neither lens is better than the other, they are made for two different purposes.

    What do you mean when you say you prefer a decent field of view to my images? Do you mean foreground to infinity to be in focus? Let us know and we can give you suggestions.

    With adding tubes and converters. This is or is not possible due to the construction of the lens and where the rear lens element is in relation to the front lens of the converter. It is nothing to do with aps-c or full frame format.

    If I have missed something or you want more clarification, let me know.

    Hope it helps.

    Gary
     
  6. Caladina

    Caladina Active Member

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    the question of liking a higher field of view i was referring to depth of field, i prefer to see more of the object in focus ie a higher ƒ stop, which seems its a trade off between more light or greater depth of view.
    of course there are times when the bokeh is important in portraits / birds etc.

    i guess what i was questioning is there a way to get an ƒ8 with as much light as a lower ƒstop , i was thinking having a larger glass might gather more light.
     
  7. GDN

    GDN Well-Known Member

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    A general rule of thumb, the larger the front lens, the better light gathering for a given focal length.

    Gary
     
  8. johnsey

    johnsey Site Moderator Staff Member Site Supporter

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    Your F stop is just the length of lens related to the diameter of the temperature. So that is a scalable calculation across any lens.
    Now regarding depth of field, my answer is yes you would likely see some variation in depth of field but not so much because of the F stop, more so the length of the lens and distance of the subject. Depth of field is also highly dependent on distance from the lens, so finding a proper comparison would be very difficult.

    With the long lens, you will tend to "flatten" the image. Think a doing a test with a 200mm portrait verses a 50mm portrait. You probably will crop the image similarly using your feet, but the image still feels different at the same f stop. I would consider the range on the lens as a simple way to gauge the f stop to use. If your shooting large format and have between f8 and f64, you probably want to shoot around f32 for a nice DoF in landscape. And use the F8 or F11 for portrait. All this said I am not one to go down the math rabbit hole others may.
     

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