September 2015 Meeting

NOCCC Energy GlobsSIG Leader: John Krill #3160

Meeting Time Is Now At 12:30 PM

We had 5 members at the meeting.

Lenses And Speed

There have been many false facts put out on Youtube on the properties of lenses and what happens when you put a lens on a camera is wasn’t designed for. This comes up mostly with Canon and Nikon lenses.

What is the focal length of a lens? Simply put it is the distance from the center of the lens to its focus point. The focal length of a lens NEVER changes no matter what sensor size. So a 50mm lens on a full frame sensor is 50mm and a 50mm lens on a APC-S camera is still 50mm. If it wasn’t it you could never focus a sharp image. Yes the apparent focal length will change but that is because the image created by a 50mm full frame lens overlaps the APC-S sensor.

What is the Speed of a lens. Though it’s called speed it is not a quantitative measurement but a ratio. The speed (N) of a lens is the ratio of the focallength (f) of the lens to the largest diameter (D) of the aperture of the lens.

N = f / D

This number NEVER changes. It is a property of the lens and not the camera body it is attached to.

Confusion comes when you put a full frame lens on a APC-S camera body. A few misguided fellows on Youtube believe you have a reduction of lens speed whenever you put a full frame lens on an APC-S camera body. NOT TRUE.

When you change f-stops on the lens the light is reduced uniformly across the sensor. Yes some of the light falls outside the APC-S sensor but that doesn’t affect the light that falls on the sensor.

Also one fellow on Youtube will tell you that ALL APC-S sensors are slower than full frame sensors because of the physical size. Again NOT TRUE. A 50mm f/2.8 designed for a full frame sensor will put just as many photons on the full frame sensor as a 50mm f/2.8 lens designed for a APC-S sensor. That’s because the lens is designed to focus all the light on their respective sensors. Both sensors get the same amount of photons period. That is just plain physics.

So just remember this. If you use a lens designed for a specific sensor then all will be fine. The only time you lose anything is when you put a full frame lens on an APS-S camera body. And that will be some of its physical focal length.

The speed of the lens NEVER changes no matter what lens, camera body combination you use.

This was verified by one of the members:

Thanks for bringing up f-stop. I tried out an old Minolta lens for a 35 mm camera on my Sony and the exposure was as you guys said. I used manual mode to compare the old Minolta lens to a Sony lens identical for APC size sensor. They produced the results you predicted, that is they’re the same. The old lens just has a bigger D and bathes a larger area with equivalent intensity of light. Douglas Stiener

User Questions

We spent most of the remaining time answering questions. It had been very encouraging that members are bringing in their questions. Keep them coming.

NEW and Exciting Developments at NOCCC

We now have our own Facebook page. To find the Club’s Facebook account just do a Google search for Computer Club NOCCC and one of the listing will be for our Facebook account. If you are use Facebook make sure you LIKE the NOCCC page.

October Meeting

I haven’t decided on a subject for the meeting. If you have any ideas just show up at the meeting and tell us what you need to know. The questions we get during the meeting really keep me on my toes.

PS: You have any ideas?

Standard Stuff That Is Always In This Report

If you have a special request for information regarding digital photography let me know. E-mail me using the address listed below.

Check out the SIG BLog for other information and maybe info on what we will discuss at the next meeting.

As always we will answer any and all questions.

If you have a special request for information regarding digital photography let me know. E-mail me using the address listed below.

See you at the next meeting.

Where the Pictures From The Main Meeting Go To Die

Note: I am using a new Web location for the photos I take at the meeting. I now upload the photos I take to Microsoft OneDrive and flickr. A link will be put here every month to the folder location with the current month’s photos.




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