IDguy Loc: Idaho

Yes, you are confused. Lens mm is independent of whether a lens projects on a DX or FX area. It does not affect the crop factor of a DX sensor.

olemikey Loc: 6 mile creek, Spacecoast Florida

The jungle canopy shooting statement (CHG CANON) should give great food for thought, light is much lower than that in the clearings. You can always enable the "crop mode" 1.3X I believe, for some added reach (at the cost of some Mega Pixels) when needed, would be better than adding more glass in between and cutting incoming light. An LED light might be helpful in certain situations, esp. at distances greater than what your built-in flash can handle (perhaps a powerful tactical flashlight would be an option, and would not weigh a lot, also useful for other thing) just remember to compensate for the color temp of the light.

If you have never shot in a canopy environment, it can be a very dramatic lighting change, like shooting outside sunny day and walking into a building to continue shooting, big difference.

was_a_guru

Not sure why you feel my Tamron 18-400 is “unfortunate”. The comparison tests I did between my Nikon lenses and the Tamron showed no discernible difference (on a 16x20 print) at every of the tested focal lengths up to 200. At 300 I had to use a magnifying glass to see any difference. With my Tamron I can carry only one lens instead of two to get a full range from 18 to 400 (or 27 to 600 equivalent based on the responses to my question earlier). I’m very happy with my Tamron.

olemikey Loc: 6 mile creek, Spacecoast Florida

I think the 1.3X crop setting will take your Tamron out to around 780 equivalent, cropped in camera, or you can always crop in PP.

imagemeister Loc: mid east Florida

Maybe because you have not compared it to a pro level lens ....

If I were going to Costa Rica for birds, I would want something "better" than the 18-400 Tammy. I would take the Tammy for back-up and general photo ops. You will need all the light you can get under the canopy. I would consider the Sigma/Tamron 100-400's but they are still 6.3 - at least 5.6 would be better. The Nikon 300 PF f4 has some merits - but I would try to avoid using it with a TC if possible.

Cropping optimized IQ is your friend.

I would also take one of these - https://www.amazon.com/Rogue-Safari-Pop-Up-Flash-Booster/dp/B00F4MOR4O/ref=sr_1_1?s=electronics&ie=UTF8&qid=1548607129&sr=1-1&keywords=rogue+safari

Personally, I would probably take this lens with TC - https://www.ebay.com/itm/Sigma-Nikon-100-300-f-4-EX-DG-HSM-lens-Sigma-1-4-Tele-Converter-B-W-Filter/182509417141?hash=item2a7e68b2b5:g:izkAAOSwTM5Y3VFb:rk:1:pf:0 - it is a great lens - I have two of them in Canon and Sony mounts.

..

rfmaude41 Loc: Lancaster, Texas (DFW area)

A 1.4 converter would take f/6.3 up to f/8.8, way beyond the f/5.6 limit of the D7500.

olemikey Loc: 6 mile creek, Spacecoast Florida

This in camera crop capability also does not change F-stop, no light loss.

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

That's exactly what I told you. With the unfortunate 18-400 you will get a equivalent field of view of 27-600 mm. You do not need a tele extender. Your lens will do just fine. No worries.

SteveR Loc: Michigan

A 400mm lens is a 400 mm lens. Remember that first of all. A 400mm dx lens, however, will only project the dx portion of the image, the center portion of what a full frame lens would project. However, those two images are exactly alike. That CENTER portion, the DX image, has a different field of view than if the image had filled the entire full frame lens.

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

It is commonly referred to as EQUIVALENT FIELD OF VIEW. And it would be 600mm at the top end of the 18-400 on a cropped sensor camera.

olemikey Loc: 6 mile creek, Spacecoast Florida

And you gain an additional 1.3X crop with the in camera crop capability, probably pushing toward an equivalent view of 780MM from that 400, when you switch from DX to 1.3 Crop in the menu. You end up with a 15.3 mega pixel image, but lose no light/no stops, just use a slightly smaller cropped section of the sensor.

It works pretty well on my 7100. Is it better than cropping in PP? ... haven't done any comparison tests yet, but the images I have made and reviewed on my 23" monitor look very good.

Would this effort be better than a 400 to 600 prime... no IMHO (and you could still crop with those lenses in camera to 1.3X, if you desire), but like a superzoom camera, might make perfectly acceptable images you wouldn't otherwise get. Is it a compromise, certainly. Would it be better to rent a top-of-the-line 400 to 600, and then crop in PP, yes.

was a guru, you have a lot to consider, and I hope we haven't confused the issue for you. Test out your 18-400 utilizing the in camera crop at 1.3X, and see how you feel about the results. it is a smaller image, but a bright image. Sharpness will depend on several factors including you and the lens. It will be harder to hand hold, but VC should really help in this case, and since no light loss, AF should work fine too, something that might let you down with a teleconverter.

Hope all this helps.

imagemeister Loc: mid east Florida

A 6.3 lens is low light is in no way "fine" for AF !

amfoto1 Loc: San Jose, Calif. USA

The Tamron 18-400mm is an f/3.5-6.3 "do it all" zoom lens. As such, it's a poor candidate to use with ANY teleconverter.

Teleconverters "cost" light, and you already don't any to spare with that lens. A 1.4X costs one stop and a 2X costs two stops of light.

Even a weaker 1.4X would make that a 25-560mm zoom with an effective aperture range of f/5-f/9 ... really dim. You'll probably have to manually focus a lot, but will have a hard time seeing to do so. A 2X would be virtually impossible to work with... a 36-800mm with effective aperture range of f/7.1 to f/13 that's almost certain to cause AF to fail completely.

Also, teleconverters "magnify" any shortcomings of a lens. They generally work best with high quality prime lenses... not zooms. As a result, image quality will take a hit, too.... Worse with a 2X than with a 1.4X... But with a zoom, especially a super wide ranging one like the 18-400mm, there will be some loss with either. How much is anyone's guess. It depends upon the particular teleconverter, as well as the lens.

Frankly, your best bet would probably be to forget the teleconverters, take the lens you've got and get closer to your subjects. On an APS-C format D7500, an 18-400mm lens "acts like" a 24-600mm would on a full frame/film camera... and that's more focal length range and reach than most film photographers ever owned in their lifetime. When I shot film for many years the longest telephotos I had were a 300mm with a 1.5X teleconverter and a 500mm that I'd never use with a teleconverter (too much loss of image quality). I made a lot of wildlife shots with those. Not just me, either. For the first ten years of his career, wildlife photographer Nial Benvie shot with nothing more than a 300mm f/2.8 lens and 1.4X teleconverter (effective combo of 420mm f/4... on film/full frame cameras). Somehow he managed to make a lot of superb images with that combo.

When using ultra long focal lengths, image quality is also effected by stability... at rock steady tripod becomes an important tool. Even image stabilization struggles at super long focal lengths.

Another thing that strongly effects image quality when using very long focal lengths is the atmosphere itself. Shooting from a greater distance with longer lenses, there's increased likelihood that humidity, various particulates, haze and heat will effect image quality.

There's an old saying among bird photographers... "You never have a 'long enough' telephoto lens." What that means is that no longer how long lens you haul around, some subjects will simply always be out of reach. If you have a 300mm, you'll wish at times you had 400mm. Once you get that, there will be times when only a 500mm will do. And when you get that, you'll need a 600mm (and one or two assistants to help you carry them all around).

Stalking, blinds, attractants such as calls or food are all ways to get closer to your subjects or get them to come closer to you.

If you can't do that or while you're waiting for a photo opp, well maybe it would be better to just put down the camera, sip on a Pina Colada, relax and enjoy the show.

was_a_guru

Thank you all for helping me understand this very confusing (at least it was to me) topic.

olemikey Loc: 6 mile creek, Spacecoast Florida

This is why I suggested he use the "in camera" additional crop (switch from DX to 1.3X crop in menu) and NO converter, and no additional loss of light/no change in lens f-stop. Like I said previously, test the "in camera crop" out on the 18-400 and see what you get, doesn't cost a thing! It is really easy, same lens, just go into menu, change it and shoot some pics, then change back to DX mode, shoot a few pics, and compare. You can even set up one of the buttons to make that change each time you push it, great for out in the field. If it is an acceptable compromise, wonderful, if not, at least you tried it.