Help understanding focal lenght and telephoto lens.

kenerickson · Jul 18, 2015

Please forgive me if this has been asked before.
I am trying to understand focal length as it pertains to DSLR, specifically a Sony A77 (crop sensor) and a Tamron 150-600mm zoom lens. My understanding of focal length is the distance from the point of convergence or optical center to the sensor measured in mm. My 150-600 when zoomed out to 600mm mounted and with me guessing the location of the sensor in only about 15 1/2 inches from the extreme front of the lens to the sensor. 600mm converted to inches = 23.6 inches. My question is how is the 600 mm derived for this lens when it does not physically come close to the 600mm?

Ysarex · Jul 18, 2015

Your understanding is correct for simple lens designs. All early lenses were designed as such. However in the 1st half of the 20th century with the rise of hand cameras and cameras that included reflex mirrors that simple lens design became a problem. This company: International renown of Ang nieux I Cinema lenses I Camera lenses came along and revolutionized camera optic design giving us first the retro-focus wide angle lens that could clear a reflex mirror and then the telephoto lens that could sit more comfortably close to the hand camera. The lenses deliver the same angle of view as their simple counterparts but manage to physically move the lens closer or farther from the film/sensor.

Joe

kenerickson · Jul 18, 2015

Thanks Joe.
Would it be fair to say then, a more accurate way to describe or compare modern "retro-focus" lenses would be in angle of view? Is angle of view a actual measured angle in degrees,minutes, seconds?

480sparky · Jul 18, 2015

Retro-focus is merely a design in the optics to allow for the lens to stay clear of the mirror mechanism, especially in wide-angle lenses. It is also used to 'shorten' the length of longer telephoto lenses.

Angle of view, or field of view, is measured in D/M/S.

To check if a lens is designed with retro-focus, merely hold it out and look through it. Make a mental note of the apparent size of the aperture. Then end-for-end the lens and look through it the other direction. If the apparent aperture size changes, it's a retro-focus design.

kenerickson · Jul 18, 2015

Thanks for the quick reply. Making a lot more sense to me now,

.
I double checked the Tamron spec page for my 150-600 and they do list angle of view in D/M/S.

And to make sure , for example, if my Tamron lens was compared to a simple lens with a actual focal length of 600mm, the angle of view would be the same for both. Correct?

Ysarex · Jul 18, 2015

kenerickson said:
Thanks Joe.
Would it be fair to say then, a more accurate way to describe or compare modern "retro-focus" lenses would be in angle of view? Is angle of view a actual measured angle in degrees,minutes, seconds?

There's nothing wrong with making a focal length comparison -- tells you the same thing -- On a FF camera a 50mm lens should have about a 40 degree angle of view. Regardless of design all 50mm lenses will have roughly this same angle of view on a FF camera. But yes angle of view can make comparisons across formats simpler. Angle of view is also frequently called field of view and is measured in degrees. I have a personal preference for wider angles of view and so if I go shopping for a lens/camera one of my requirements will be a lens on the camera that makes it to at least 74 degree angle of view -- measured in degrees against the horizontal side of the film/sensor format. (Don't confuse angle of view with angle of coverage). On a 35mm FF camera that would be a (roughly) 24mm lens and on a APS format camera that would be a (roughly) 16mm lens. On the tele end you're talking single digit degrees up in the 600mm range.

Joe

Derrel · Jul 18, 2015

kenerickson said:
Please forgive me if this has been asked before.
I am trying to understand focal length as it pertains to DSLR, specifically a Sony A77 (crop sensor) and a Tamron 150-600mm zoom lens. My understanding of focal length is the distance from the point of convergence or optical center to the sensor measured in mm. My 150-600 when zoomed out to 600mm mounted and with me guessing the location of the sensor in only about 15 1/2 inches from the extreme front of the lens to the sensor. 600mm converted to inches = 23.6 inches. My question is how is the 600 mm derived for this lens when it does not physically come close to the 600mm?

A"telephoto" lens design uses optical design principles that allow for a lens whose PHYSICAL length is actually SHORTER THAN the focal length. The shorter the lens is in relation to the focal length was at one time referred to as the telephoto factor.

The OLDER lens designs with focal length and physical lengths being the same are now properly called long focus lenses; good examples of long focus lenses are the low-cost 500mm f/8 and 400mm f/6.3 lenses that have been around for decades; simple optical formula, three, or four elements, in a loooooong, skinny barrel, and retailing for $109 these days. Many 500mm f/8 pre-set diaphragm long focus lenses have been sold under store brand and importers' names, going back to the late 1960's. I own one. It's a looooong, skinny tube and is pretty light in weight.

Catadioptric lenses use internal mirrors to "fold and reflect" the light, thus achieving long focal lengths, like 300mm or 500mm, in VERy short overall length barrels. I have a 300mm f/5.6 Celestron catadioptric lens that is approximately 105mm in total overall length.

The Tamron zoom has a pretty significant "telephoto factor", achieving its maximum 600mm focal length in just under 16 inches of total overall length.

wfooshee · Jul 19, 2015

Basically, telephoto doesn't really mean "long focal length" the way it is commonly used. It means something along the lines of "artificially lengthened focal length." The lens doesn't actually have to be 200, 300 or even 600 millimeters away from the imaging surface, as the optical design pulls the actual focus point much closer in.

Similarly, wide angle lenses are usually retro-focus. That means the focal point has been optically extended from the lens body, so a 10mm lens doesn't have to actually be 10mm from the imaging surface.

These two factors are the main reason that 50mm lenses are so much cheaper than other focal lengths, even with much larger apertures. A 50mm lens requires almost no adjustment for the actual distance from the imaging surface, so no "weird" compensating elements to move the focusing point nearer the lens or farther away.