[zoomlens]

I have read the descriptions of the ratings and understand what they are saying. However, my simple mind still does not comprehend something.

For nature and wildlife photography a 400mm or larger is often quoted as being necessary. Then when I look at the lens for sale, the listing confuses me. For example: A 300mm lens will then say 35mm equivalent of over 400mm. So, would I be getting the size that is needed or should I just forget the 35mm equivalent ratings and make sure that it is 400 or over?

Sorry if this is so simple and inane to some of you, but I want to look for what I need and not assume something into the sales talk that doesn't matter.

[boriscleto]

Just forget you ever heard the term "35mm equivalent".

[MadMathMind]

Just forget you ever heard the term "35mm equivalent".

This.

The equivalence is worth thinking about only in two situations:
1) You are comparing lenses on APS-C from Canon to any other brand (because Canon has a 1.6x CF while everyone else has 1.5x, so this is mostly Canon's fault).
2) You are moving to or from full frame and need to figure out how to reorganize your lineup.

You're doing neither so best to forget this 35mm nonsense.

[pepperberry farm]

Just forget you ever heard the term "35mm equivalent".

ditto....

[marcusBMG]

Concentrate on field of view, and magnification. Useful:

Angle of View Calculator

Magnification is easily estimated by starting from the focal length that ~ = "as you see" ie 1x. On pentax that's about 33mm. So 400mm would be about 12x. The more magnification, the closer you can bring your subject but the more difficult the photography. Most work with 300mm-400mm.

PS you initially started asking about lens ratings. These need to be taken with a pinch of salt, and certainly not as any sort of absolute measure, but in the aggregate do identify the better lenses.

[stevebrot]

For nature and wildlife photography a 400mm or larger is often quoted as being necessary.

For APS-C size sensors (all current Pentax dSLRs), conventional wisdom is that 300mm is the shortest reasonable focal length for bird and wildlife photography. For FF size media (35mm film, some dSLRs) 400mm is often considered to be the minimum. For both, the focal length is the number written on the lens.

In regards to crop factor or focal length equivalence, it is something best not thought about for photographers under a certain age. The sole exception would be people who have a previous history with 35mm film photography who have an existing intuitive sense of FOV for a particular focal length on that format and for which a quick calculation is helpful.


Steve

(...wishes the terms "crop factor" and "35mm/FF equivalent" would simply go away...)

[Adam]

I have read the descriptions of the ratings and understand what they are saying. However, my simple mind still does not comprehend something.

For nature and wildlife photography a 400mm or larger is often quoted as being necessary. Then when I look at the lens for sale, the listing confuses me. For example: A 300mm lens will then say 35mm equivalent of over 400mm. So, would I be getting the size that is needed or should I just forget the 35mm equivalent ratings and make sure that it is 400 or over?

Sorry if this is so simple and inane to some of you, but I want to look for what I need and not assume something into the sales talk that doesn't matter.

Always just look at the focal length printed on the lens itself, and forget about equivalencies.

This might also help:
The Crop Factor Unmasked - Articles and Tips | PentaxForums.com

Adam
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[lytrytyr]

For nature and wildlife photography a 400mm or larger is often quoted as being necessary. Then when I look at the lens for sale, the listing confuses me. For example: A 300mm lens will then say 35mm equivalent of over 400mm. So, would I be getting the size that is needed or should I just forget the 35mm equivalent ratings and make sure that it is 400 or over?

Equivalence is important here, and cannot be dismissed.

If the "400mm or larger is often quoted as being necessary"
appears within the context of "full-frame" 35mm photography,
say by respected wildlife photographers working with full frame cameras,
while you are planning to apply that advice to APS-C photography,
you _will_ need to understand "400mm" as a 35mm equivalent,
and then interpret it as an actual focal length of around 250mm
when you go shopping for a suitable lens.

A lot of people wish equivalence would go away,
and it can get confusing at times,
but it's a fact of life whenever the discussion involves different formats.

My 90mm Apo-Lanthar is a long way from 400mm in actual focal length,
but it's my lens of choice (within the Q system) for what little casual wildlife photography I end up doing.
Why? Exactly because it's a "400mm equivalent"!

[zoomlens]

You have verified one thing that I had about decided, but with the various posts, I get a better (at least for my simple mind) understanding of things. And the chart and associated article was interesting and helped a lot.

[BrianR]

A tool for a visual comparison of focal lengths on 35mm and aps-c (their aps-c isn't an exact match for our Pentaxes, but it's close enough):

Focal length comparison tool, Tamron USA

Ignoring the term "equivalent 35mm" makes sense if you never plan to shoot anything but your aps-c and have no interest in discussing focal lengths with anyone who shoots anything other than aps-c.


Even once you've pinned down the format, statements like "For nature and wildlife photography you need 400mm or longer" are very subjective. Not everything that falls under "nature and wildlife" flees when you enter the same county. Figure out what you want to photograph, how large it is, and how close you can expect to get, then head to the calculator linked to above for an idea of the focal length you'd need/want.

[stevebrot]

it's a fact of life whenever the discussion involves different formats.

Do tell. Unless the inquiry is being made by a former 35mm film photographer wanting lens purchase guidance, the notion of equivalence or crop-factor is rather meaningless regardless of how many formats are being considered. I shoot APS-C, 24x36mm, 6x6cm, 6x7cm, and 4x5" formats and "equivalence" is never a consideration except when taking part in discussions like this. I will repeat for emphasis...never.

That being said, field of view often is, but I don't waste a lot of energy normalizing the other formats to 24x36mm as if it were some sort of standard for reference.* Strangely this is despite having deep experience with 24x36mm (FF) going back to 1968. With medium and large format photographers, crop factor is almost never a topic of discussion, though the common categories of "wide", "normal", and "long" often are and the conventional focal lengths for all three are well-known.

Thinking in descriptive terms, below is a good cheat sheet for APS-C:

Ultra wide-angle: shorter than 16mm
Wide-angle: 16mm - 24mm
Normal (as a range): 26mm - 40mm
Portrait/Short tele: 45mm - 85mm
Moderate Tele: 90mm - 140mm
Long/Sports Tele: 150mm - 300mm
Ultra/Wildlife/Bird Tele: 300+ mm

Another approach is to simply note that the Pentax kit zooms span the range of 18-200mm allowing coverage for all but the extremes of short and long with focal lengths outside that range being special purpose. There is a third and rather dumb approach; that being to simply consider what focal lengths are most commonly offered for the format in question. That is what I did when assembling my 4x5" kit.


Steve

* It is possible to do so, but utility is limited when aspect ratio is other than 2:3. For example, a 90mm on 4x5" tends to shoot much "wider" in practice than its diagonal FOV would indicate.

[Sandy Hancock]

To be fair, the OP has it pretty much right from the outset. A 300mm on a crop does indeed give the same sort of *reach* as a 450mm would on Full Frame. You get the same magnification and perspective with a considerably smaller and lighter lens.

The depth of field measurebators will then hurriedly point out that is not fully *equivalent*. That's where boriscleto's approach should be adopted; ignore them like the vast majority of us do.

[wildman]

I want to look for what I need

In that case what you "need", especially for wildlife photography, is an absolute not a relative measurement.
FL is relative - 600mm is, all else being equal, twice as powerful compared to a 300mm but says nothing about absolute power.
Magnification does.

Assuming you know you will need, say, 10x for your field work then just multiply mag needed by sensor diagonal and you will know, at least pretty close, what FL lens you should be looking for.
Thus 10 (10x magnification) x 28 (28mm -the diagonal of a APS-C sensor) = a 280mm lens on a APS-C body and about 430mm on a FF.

It's not rocket science.

[stevebrot]

Assuming you know you will need

Aye, there's the rub...

Folk have a hard time visualizing "n" degrees field of view or powers of magnification. At least, I do.


Steve

[wildman]

Folk have a hard time visualizing "n" degrees field of view or powers of magnification. At least, I do.

After decades of using every manner of optical device for just visual use (not photographically) I think I have developed a sixth sense about what FOV to expect from a given FL.


Pushing things to their limits. Shot through the 560mm with the original Q on the back end. With that tiny sensor that would give me 73x mag.
Actually it was an easy shot. I was just sitting at the kitchen table having my morning coffee when I took this through the open back door - really roughing it.