Fixed "f" ratio zoom lenses, don't be fooled

[Lester]

Hi all,

I have a Canon 70-200mm F2.8 L lens with front element of 77mm. I had thought that this lens would be 77mm at all focal lengths, but it seems not. When one increases the magnification usually the f ratio will alter and light is lost, so a longer exposure is needed.

In this case as the lens is zoomed in (increased magnification) the f ratio stays constant. There must be a trade off. If the f ratio stays constant then something else must alter. To my knowledge the only thing that can alter in this case is the diameter of the front lens being used.

200mm f2.8 = 71.42mm max diameter
70mm f2.8= 25mm max diameter

does that startle some of you; It did startle me. A few days ago I took images of Comet Lovejoy with the lens at 70mm with same exposure as I had been using when at 200mm. Both are at f2.8, but the 70mm image hardly revealed the comet at all. At 200mm it was much easier to see.

When you think about it there is alway a trade off; if the f ratio alters as one increases the magnification then the diameter of the front lens can stay constant, But if the f ratio stays constant then the diameter of the front lens, Must alter. I did a google search and found articles that back up my thoughts. I have never heard this talked about, but many of you may be well aware of it.

With this in mind, I'd say constant f ratio zoom lenses are not necessarily the best for astro photography, where maximum diameter of the front lens is needed for faint objects. I would be interested to hear what the more learned than I have to say on this. Link to one of the searches I did. All the best.:thumbsup:

I have reworded what I have written to help get my point across more clearly.

http://www.thephotoforum.com/forum/beyond-basics/219987-constant-f-stop-zoom-how.html

[Lester]

Due to this finding, I have now purchased a 85mm f1.2 lens to overcome the shortfalling of the 70-200 F2.8 lens.

[rally]

Lester,

The f-ratio actually considers the virtual aperture of the lens which is not necessarily the physically measured aperture of the front element.

So your zoom lens will have a constant physical aperture, but a varying virtual aperture, that varies relative to the focal length, hence making it a constant f-ratio zoom lens.

I think the confusion lies in what the definition of "aperture" actually is.
In a Telescope its one and the same (front element diameter) but not so for a camera zoom lens.

I am sure others can elaborate further and better than I.

[Lester]

Exactly, you have said it well. When the actual aperure gets smaller it reduces the lens ability to record extended/diffuse astro objects.

All the best.

[troypiggo]

Careful with the 85L for astro. This lens is not full time manual focus. You have to push the shutter down half way to "activate" its focus ring. Doesn't make it particularly condusive to astro imaging IMO. If you don't have the shutter pressed half way, you can spin the focus ring as much as you want and nothing happens. It is by design, so not a fault.

That being said, fantastic lens. My favourite at the moment if you base it on amount of images taken with it over my other lenses.

For astro and around the same focal length, I'd go with the Tamron 90 macro lens. It does not just have to be used for macro. It's wonderfully sharp, much, much cheaper than the 85L and other macro lenses around the same focal length. But best of all, it has (IIRC) 9 aperture blades and they're curved. What this means is that even stopped down a little, it doesn't produce massive diffraction spikes like you get with other lenses.

[Lester]

Thanks for the info Troy. That was news to me. I should have it by mid next week.